Vacuum increases after initial warmup

[joecal]

Hello,
I've noticed the 305ci engine in my 86 after warming up say 10 minutes or so while idling before driving it seems to run smoother and the vacuum increases to about 19 and holds steady there which is fine. I have the timing set at 10-12 degrees btdc which it seems to run good there. I just replaced the carburetor with a professionally rebuilt one, the same Quadrajet part #17085226 that was on the truck originally. They blocked off the electronic mixture control solenoid because all of my emission parts where removed by the PO so it was running too rich I think because of that solenoid. It was using a lot of fuel and seems better but I can't say for sure just yet. So my question is why is it running smoother after a long warm up period? It actually did this before I changed carburetors so I'm not sure if it's the carburetor doing this but maybe it is. Any insight would be appreciated.

 

[HotRodPC]

You might have some dirty valve seats or getting close to needing a valve job. Is the needle erratic when cold too? As the engine warms the crud on the valve seats is more pliable or the metal faces are seating better.

You're in Cali, so I know you're running an EGR valve. Could also be a dirty EGR valve that isn't seating or sealing off real good. That should be taken off and cleaned every couple to 3 years anyway. I'd start with that.

 

[joecal]

You might have some dirty valve seats or getting close to needing a valve job. Is the needle erratic when cold too? As the engine warms the crud on the valve seats is more pliable or the metal faces are seating better.

You're in Cali, so I know you're running an EGR valve. Could also be a dirty EGR valve that isn't seating or sealing off real good. That should be taken off and cleaned every couple to 3 years anyway. I'd start with that.

Thanks for the reply, the needle is not erratic when cold or warmed up it holds steady. I'm not running an EGR valve, all the emission parts were removed by the PO, I wish he would have left it alone. I'm not in Cali, you probably saw my login name joecal and thought that,lol. I'm in Illinois and the truck being 30 years old, I believe it's exempt from pollution testing and I have antique license plates on it.
Thanks again.

 

[HotRodPC]

Guess I fuct that all up and got caught skimming threads again.

 

[89Suburban]

Guess I fuct that all up and got caught skimming threads again.

 

[chengny]

I've noticed the 305ci engine in my 86 after warming up say 10 minutes or so while idling before driving it seems to run smoother and the vacuum increases to about 19 and holds steady there which is fine.

So your basically asking why your carbureted engine (without it's ECM and associated controls) runs smoother - and also shows increased manifold vacuum - when warm?

That's just the way it is.

Ah - just ******' with ya! Maybe this will shed some light on the extended warm up/smooth idle times but it sounds like you already know why:

The ECM determines proper air/fuel mixtures for engine operating conditions by monitoring oxygen sensor and other vehicle sensor signals. Air/fuel mixture is controlled through the MC solenoid by varying the amount of time the solenoid is energized (dwell period) during each cycle. This is called ``Closed Loop'' operation. During some modes of operation, such as when engine is cold or operating at wide open throttle, the dwell period will remain as a fixed value. This is called ``Open Loop'' operation.
MC solenoid operation can be monitored using a dwell meter connected to the MC solenoid test lead and set on the 6 cylinder scale for all engines. Normal dwell reading during ``Closed Loop'' fluctuates between 10°and 50°. If leaner mixtures are required, reading will fluctuate toward the higher end of the scale. If richer mixtures are required, reading will fluctuate toward the lower end of the scale. For proper ``Closed Loop'' operation, the amount of dwell fluctuation is not important, but the reading must fluctuate constantly.
During normal idle and part throttle operation, this system operates in ``Closed Loop.'' However, under certain operating conditions the ECM sends a fixed dwell signal to the MC solenoid, and operates in ``Open Loop'' mode.
During cranking below a certain RPM, the system is in ``inhibit'' mode and the dwell signal will remain fixed for up to 25 seconds, then drop to zero until a specified RPM is reached. During engine warm-up or when oxygen sensor is cold, the dwell signal will remain fixed until the sensors reach specified operating temperature. When engine is operated at wide open throttle, the Throttle Position Sensor (TPS) sends a higher voltage signal to the ECM, which provides a fixed enrichment signal to the MC solenoid.

So, when the carb shop blocked the flow of fuel through the MCS, they locked the F/A ratio - that would normally be tweaked richer during cold conditions. As a result, they also increased the time to reach normal operating temperature. Internal combustion engines run much smoother at normal operating temp.


BTW - if you run an MCS equipped Q-jet without any control input from an ECM, it defaults to full rich.

So you are right on that - without the rest of the stuff that goes along with it, an MCS Q-jet does burn a lot of gas. But you can't just disable the MCS ports and expect increased MPG without affecting factory warm up performance.

You can check on this, but trust me, I have tried - a lot of times - and it is true. Unless your carb shop did some extensive modifications, in addition to just the MCS removal, it is running leaner than it should be. And it goes without saying - at certain times (e.g. during warm up & WOT) - you pay for that lower F/A ratio . There's no such thing as a free lunch.

As far as the manifold vacuum mystery:

You don't mention whether your RPM increases as the engine approaches operating temp, but I'm sure it does.

This is due to all the basic reasons (more efficient atomization of the fuel, warmer intake air, more complete combustion, warmer lube oil, etc). But for what ever reasons, it does run faster. And - as long as the choke plates aren't sticking and you just leave the throttle plates alone (i.e closed) - as the pistons begin stroking faster, they will try to pump more air. But with the plates closed, they can't get it.

I know you can see where this is headed. Starved for air flow on the intake stroke, the increased speed of the pistons will pull a deeper vacuum in the intake manifold.


This are only my opinions, so consider the source. But I think I'm pretty close.

 

[chengny]

Jeez, dude a Nutkick. Wow. He's the frickin' administrator and you're only a super mod. Man, I wish I had your balls!

 

[joecal]

Thanks chengny for that thorough explanation, I appreciate you taking the time. You are correct that the idle does increase a little when it fully warms up. I just happened to have a vacuum gauge on it at the time and noticed the vacuum rose to where it should be. I was going to adjust the timing again because the gauge read late ignition timing say at 15 degrees or so until the idle increased a little and then it read fine in the green around 19 degrees. I probably should have went with a non-electric carb as a few people on this forum suggested to do, but the carb. shop said it would have the same affect by blocking the MCS and it would still look stock, not that I care about that. I didn't want to pay a core charge if I decided to keep my old carb or sent a different one back to them.

 

[chengny]

You know Joe - don't get too bummed out yet.

Are sure you even really have an MCS type metering system - and not a dual capacity pump type? In 85/86 the pickups pretty much all got dual capacity pumps. To run an MCS, the ECM needs inputs from a TPS, an 02 sensor and usually also an idle control solenoid output. It looks like this:

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The dual capacity pump type is easily modified and it may well be that is what your carb shop was talking about. Even unmodified they still perform well, that's probably why they were speced for the truck line. If your cap was some other color than blue, the carb may be perfectly fine (probably is) and is not the problem. This might be a simple choke issue or an air leak that seals itself when things warm up, for example.

The problem is, these trucks used THREE Q-jet versions in '85/86 based on destination/GVW. (apparently)

Just because a Q-jet has an electrical plug does NOT mean it is CCC, ECM, Feedback, whatever you want to call it. If it doesn't have TPS and O2, it likely won't have an ECM. ESC yes, but that was standalone before injection, except *probably* on TRUE CCC equipped trucks, which are apparently quite rare.

You are correct that a true CCC carb will run full rich in "fail" mode, but a one plug Q-jet does not operate the same way as far as I've been able to discern, it's still got a power piston that is vacuum controlled, but is limited during warm up.

You know, just go here and read this thread. It's got a happy ending for the kid. At first some colorblind ******* told him that he had just wasted his time and money. Because the guy mistook the DCP cap for an MCS cap, and the poor kid didn't have an ECM/CCC - the guy basically told him he might as well throw his new carb in the trash. Sucka!

http://67-72chevytrucks.com/vboard/showthread.php?t=595082



More stuff I had in my Q-jet folder:

How to ID a Q-jet with a dual capacity accelerator pump:

The dual capacity accelerator pump carbs can be ID'd by their solenoid plug mounted on the passenger side top front of the airhorn/carb top, along w/the LACK of a plug for the TPS (throttle position sensor) that's present on electronic feedback-type Q-jets used during the same years on vehicles using the "CCC" (Computer Command Control)emissions system.

I did some checking and there are a couple different Q-Jets used on an '85 Suburban. One is the 02 Feedback system I mentioned before. The model #'s I have for that are E4MED & E4ME. The first letter "E" designates Computer Command Control system (i.e. 02 feedback) and the other "E" designates electric choke. The other one I have listed is one that still has wires coming out of the top. That one is an M4MC, M4ME, M4MED, or M4MEF. These carbs have what is called a dual capacity pump solenoid. When the engine is cold, more fuel is necessary to ensure a smooth transition from idle to part throttle. When the engine is warm then less fuel is needed. A coolant temp sensor energizes the pump solenoid when coolant temp exceeds 170 deg. F. The solenoid opens the dual capacity pump valve, which reduces the pump capacity. It's possible if you have the second type that when engine is up to operating temperature that it would run fine. I couldn't see how it would run right if you had the first type (with the mixture control solenoid) as those could be pretty tweaky at the best of times and without the proper inputs from computer won't know how much fuel to meter. You’d be running extremely rich. Nickmil.

These NON CCC/NON electronic dual capacity accelerator pump Q-jet carbs are found on 1985-'86 GM trucks and vans, and have desirable features- adjustable top-accessed APT, electric choke, correct linkage geometry for OD transmission TV cable, large 800 cfm casting, and generally less use/abuse/rebuilds on them than carbs from the '60s and '70s.

How to make it work: Basically nothing needs to be done to have the carb perform just like the earlier single capacity accelerator pump carbs. The plug and solenoid can be left intact, the default setting (with no external wiring or electrical source supplied to the plug) is such that the accelerator pump will deliver a normal pump shot. Or if desired the cavity under the solenoid can be filled with molten lead to prevent any chance of a leak.

 

[chengny]

Now do you see what I mean by this:

This are only my opinions, so consider the source. But I think I'm pretty close.

Anyway, back to the rough idle at start up (I think we can forget about the slight vacuum change):

Have you had a chance to check the lash/pre-load in your valve train? If you are still running the original lifters they are probably pretty sloppy after 30 years.

How about the ignition system? Before I tore my current truck down for restore, it was running like crap. Not always - sometimes it ran fine - and not only when cold like yours. I could smell that it was running rich, so I was sure it was a excessive F/A ratio. So then, for like 3 months, I ****** around with carb after carb. I wasn't using anything new or even rebuilt, just 3 or 4 decent Q-jets I had laying around (had worked very well when put on the shelf). Even rebuilt a motor and dropped it in. None of those carbs could provide a consistently smooth idle - and it was getting worse. Then one night I drove over to the DQ to get something for my wife and on the way the truck ran so bad it felt like it was going to actually stall. I crawled under that ****** with the motor running (it's a big high truck) and told myself I was going to stay there until I figured this problem out.

Didn't take long till I saw the problem - mainly because it was dark and the hood was closed. The area between the LH side of the engine and the inner fender was lighting up like the 4th of July. The wires to both the 5 & 7 plugs were arcing out nearly every time they fired. And these were those expensive yellow...what Accel? wires that cost me nearly $100. Damn things weren't even two years old - and I seem to work on my trucks more than I drive them, so they had not had a hard life. Went home and put the stock Delco wires back on and the GD truck immediately ran like a limo. And guess what else, because the fuel in those two cylinders was now being combusted rather than going straight out the exhaust pipe, the gas smell disappeared along with the rough idle.

Sorry, sometimes it takes me awhile to get to the point. I could have just said "Get under the truck while it's dark out and check for arcing from your wires to the block. The loss of ignition in even one cylinder will cause rough idle and a raw fuel smell".

Anyway, what other wisdom can I offer? Since the carburetor was just recently overhauled by professionals, I guess we can assume that the choke plates are getting set to the closed position. And you seem to know how to set a choke by pressing the accelerator to the floor one time before cranking. Don't try to warm the engine faster by cranking down on the choke pull, it won't help. You might as well just sit in the truck and rev the engine with your foot, that'll work (see below). Even on the coldest NH mornings, if everything is okay, the choke plates should immediately begin to open as soon as the oil pressure comes up to 10 psi - and the motor should still idle smoothly. Fast idle speed cams are set properly, right?

Your coolant thermostat is the right temperature rating - and operating properly?

I can't think of any fuel delivery problems that would seem to (only) effect idle performance when cold.

Air leaks under the throttle plates and into the manifold? Maybe.

Air leakage generally increases idle speed but it's a false indication of increased performance. If the engine runs faster, it must be working better right? Nope, it's running faster only because; with the leaking in of the air (which is free to flow past the closed throttle plates), the pistons have less vacuum to fight against on the intake stroke. Also due to the reduced vacuum across the idle jets the F/A ratio falls off. And with the reduction of fuel available, less heat is generated within the combustion spaces. I would almost say to look closely at your intake manifold/cylinder head joints (and any other associated hoses or gaskets) for leakage. But that doesn't sound right. I am certainly no expert, but in my experience I haven't come across any air leaks that consistently seal up as the engine warms.

Let me make sure I have this right. It goes like this:

1. You go out on a cold AM and crank it over. It starts right up. But it idles poorly - poorly enough that you have to wait 10 minutes to drive it.

2. So for those 10 minutes, you just sit in the cab - or maybe open the hood, lean over and observe (either way, without revving the engine) and patiently
wait until it reaches operating temperature. Or at least until the needle on the coolant gauge starts to come off the low end.

3. As you wait around for it to warm up, the idle starts to smooth out (and the rpms to come up a bit). Still, without any help from you.

4. At some point; either from experience, the smoothness of the idle, the reading on the coolant temp indicator, or even the slight increase in rpms - you
decide the time is right to strike.

5. So then you finally hop in, drop the gearshift into drive and head off to work. And the truck runs great all the way.


I gotta tell you buddy, you're a better man than I. If it were me in your shoes, I would sleep for those extra 10 minutes. Then I'd go out, crank that pig up and rev the **** out of it's lazy ass till it ran right.

 

[joecal]

Hi chengny, thanks again, I know that was very time consuming. I must have a dual capacity pump carburetor because the mixture control solenoid is a grayish color and there is no TPS on it. I haven't checked the valve lash but I did replace the cap, rotor and wires on the truck a few weeks ago. It's really not idling rough, it's just I noticed the idle increases slightly after warming up and it sounds smoother. I took it for a short drive yesterday and it ran okay but still seems like it's using more fuel than it should be. Thanks again for your help, it's much appreciated!

 

[chengny]

Alright man, good for you on the carb. I hate doing shotgun repairs at all, but especially when it involves the carburetor. Even though you can flip in them and out in an hour or two, it still sucks because of the uncertainty of what you are installing. If you have a lot of disposable income, it's not an issue. You just go on ebay and buy yourself a nice NOS Q-jet - and be fairly certain that you will have eliminated the carb as a possible poor performance cause.

But wife keeps me insulated from my disposable income (and I'm also a cheap ****). So that isn't an option for me. That leaves like 4 options:

1. Like you, have a reputable local shop go through what you already have. Then due to the fact that people still bring them carbs to rebuild you can trouble shoot with confidence. I'm too cheap for that.

2. Get a rebuild kit and a good manual. Then lock yourself in your shop for about 3 work nights or an entire weekend and DIY. In the end, you'll know it was done right (if you do it right). Also, you will actually know what you're talking about regarding carbs. A lot of people don't know what they're talking about, but still talk like they do. I'm not patient enough for that one - or maybe too lazy.

3. Drive around to any and all the junkyards in your area searching for one that looks decent from the outside. This is getting harder to do. There aren't that many pre-87 GM trucks left in the yards - at least in New England. But on the upside, if you spend a lot of time in the yards pulling your own stuff, occasionally you can strike gold. And you can get it cheap. That method has always been my favorite. I spend the day and bring a small cooler of beer. Also I try to always bring a big bag of Chinese food for the counter guys. Do that a couple of times and you'd be surprised how fast prices for your parts drop. I'll let you in on a secret; unless it's a big item (e.g. motor/tranny) or has a VIN on it, the counter guys are pretty much allowed to just make the prices up. And as a bonus - I don't have to bring any tools. Seems they have plenty and are happy to share.

4. Look for wicked bargains on CL and ebay - they do pop up. But you will have to spend many hours searching. And if you want something, be quick. Ideally, you should be retired for this method.

Below is what started me writing this - cause we were talking about carbs. I had just finished a 350 rebuild and it looked real good. I wanted to have a tight carb on it right from the start - more as comparison to other engines I had done than anything else. Anyway found this for $90. The story I got was it was bought as part of a restore that went sour - maybe on the engine for an hour, blah blah blah. Dude, I already said I'll buy it - Jeez.

I wasn't ready to fit the new motor yet but I was dying to see if I got burned. NOPE - best $90 I ever spent. Put it on the motor that was coming out and test drove. I was shocked. Apparently I had never had a Q-jet that did exactly what it was supposed to do. I hadn't touched a thing on it. Just bolted it on, connected the gas line/throttle cable/hoses and cranked the engine. It was a remarkable difference and I was pleased with the old carb. Acceleration was incredible. Coming off idle, there wasn't even a hint of hesitation or stumble. But maybe the best was warm up. Even on a 10 degree morning, the only thing I had to do was give the gas pedal a single punch to set the choke. After that, the carburetor took control and did everything right - no matter how cold it was. I was amazed.

The point of this whole story is: if your carb (which you now know is the correct application for the engine/truck) was done properly and your truck doesn't run as described above, your warm up problem lies elsewhere. BTW don't worry about my time spent writing. I just turn to my laptop during the commercials and write a few sentences. Plus when I write something, I want what I say to be correct. So I do research, and that usually makes me learn something as well:

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[joecal]

Thanks again chengny, that's some good advice about junk yards. I do need a few things like the cruise control rod for the servo, I guess it's not available anymore new. That's pretty cool you make a day of it beer included,lol. I don't have disposable income like I used to, you would think the older I got I should have more money. Oh well ,old cars and trucks are a hobby that I enjoy. That's a great find on the carb and a great price. I've been watching Redneck Restorations This Old Truck on youtube, Jeff Bradshaw is a pretty knowledgeable guy and fun to watch. Reminds of an old friend of mine that restored mostly MG's, but just about anything.

 

[chengny]

ROD. Cruise Control. W/Washer.

Part 14009160

I think these guys are for real (63 in stock):

Model: 14009160
Shipping Weight: 0.1lbs
Units in Stock: 63
Manufactured by: GM
$28.86


http://www.discontinuedspares.com/index.php?main_page=product_info&products_id=1701789

 

[yevgenievich]

Sorry to plug advertisement in, but I have a complete spare cruise control unit for sale