Engine runs poorly spontaneously?

[Sad Sack]

Have done a vacuum measurement on PCV port value = steady 16inHg at high rpm, see picture. Engine runs very poorly and make sucking noise when I let it run stationary, I make movie but can't attached it? Vacuum value is very low 4inHg

Smoke test, thorough visual inspection of all vacuum related hoses, gaskets, spraying carb cleaner around areas associated with vacuum etc.
Some cases a leak will be obvious by the "hissing" sound.

How To Smoke Test For Vacuum Leaks: A Step-by-Step Guide

Learn how to smoke test for vacuum leaks with our comprehensive guide. Find and fix engine problems with this essential diagnostic technique. Get step-by-step instructions and safety tips.

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[NLD79K10]

4inHg is very low and indicates a pretty significant vacuum leak somewhere. Start with checking out the assorted vacuum hoses. You may be able to pinpoint the area that the hissing is coming from by using a short piece of hose as a makeshift stethoscope by putting one end close to your ear and moving the other end around until you have a general area where the hissing is loudest. If that doesn't get a result, you're going to have to do a smoke test. Going by the low reading, I'd suspect a large split in a hose somewhere or a hose that is completely disconnected. Makes me wonder whether you hade a serious backfire when running on LPG at some point.

 

[motionz]

First of all, I pinched off the following vacuum hoses:

- brake booster
- EFE and PCV
- all hoses from the Evap Canister; there is still a lot of California emission stuff on them

Next, I sprayed brake cleaner around the carburetor, and on the right side (passenger side), the engine responds immediately and revs higher; see photo. What would be the most likely leak, or something I can do temporarily for the test?

I haven't had a backfire, but I did hear a pop multiple times when the engine stalled.

 

[Hunter79764]

Try the trick with a piece of vacuum tube to your ear. It looks like there's multiple gaskets that could be leaking there, fix that and I bet you will be running smooth again.

 

[NLD79K10]

There's a couple of potential vacuum leak options in the area of that red ellipsoid. Could be the gasket of the EGR blank-off plate (looks like there's just a bit of thin sheet metal behind the spacer rather than a nice thick blank-off plate that can provide decent rigidity for sealing), the hot air-choke plate gasket, carb base gasket or even the throttle shaft itself. you may be able to narrow it down by using a straw on the nozzle of your brake cleaner can and small squirts to more accurately spray in specific areas.

 

[motionz]

I have since connected a timing light next to the vacuum gauge so that I can check these parameters with every change.

Spraying brake cleaner caused the engine to higher rpm: 1300rpm, 30 degrees, 15inHg; a few seconds later, it drops back to 400rpm, 18 degrees, 4inHg.

After the search involving spraying brake cleaner, I checked a number of items and replaced them where necessary:

- checked the EGR valve; it is completely sealed
- fitted the choke housing with new O-rings around the shaft and replaced other parts
- replaced the hot pipe choke gasket

The engine continues to run poorly: 400rpm, 18 degrees, 4inHg. When performing the test with brake cleaner again, the engine barely responds, so it seems to be resolved.

In the meantime, I replaced the HEI module and have now ordered a new ignition coil and spark plug wires.

 

[Sad Sack]

Don't forget to check the vacuum hose elbow for issues at the transmission modulator.

 

[Ricko1966]

I have since connected a timing light next to the vacuum gauge so that I can check these parameters with every change.

Spraying brake cleaner caused the engine to higher rpm: 1300rpm, 30 degrees, 15inHg; a few seconds later, it drops back to 400rpm, 18 degrees, 4inHg.

After the search involving spraying brake cleaner, I checked a number of items and replaced them where necessary:

- checked the EGR valve; it is completely sealed
- fitted the choke housing with new O-rings around the shaft and replaced other parts
- replaced the hot pipe choke gasket

The engine continues to run poorly: 400rpm, 18 degrees, 4inHg. When performing the test with brake cleaner again, the engine barely responds, so it seems to be resolved.

In the meantime, I replaced the HEI module and have now ordered a new ignition coil and spark plug wires.

Some brands of brake clean when burned create phosgene gas( very serious health hazzard) including death. Use carb cleaner. I would bet you money your problem is not ignition related with the possible exception of a ruptured vacuum advance canister. Disconnect and plug the hose at the egr valve see what that does for you. Remove the pcv valve from the valve cover, tape over the opening for the pcv valve and tape over the opening for the valve cover vent on the opposite valve cover. Put a vacuum gauge on the dipstick tube. If you are pulling a vacuum there, you have an intake leak on the bottom side of the manifold.

 

[motionz]

I have checked the transmission modulator also with brake cleaner, no problems.

The vacuum advance I have allready checked with a vacuum handpump, movement is oke and no leakage. EGR valve is blocked with a plate. The test with cover the two valvecover openings and the vacuumgauge on the dipsticktube no vacuum measured, engine still runs @400rpm, 18 degrees, 4inHg..

Tomorrow I wil replace the ignitioncoil

 

[NLD79K10]

Since the engine barely changed revs when adding extra hydrocarbons to the mix, it seems that there's no vacuum leaks that are big enough to affect the idle this badly.
Is that 400 rpm on LPG, petrol (gasoline for our readers from across the pond) or both? It this with engine warm or cold?
Have you had a look at the state of your spark plugs to see whether the mixture is too rich or too lean?
What's the compression on the engine like?

 

[Johnny Atomic]

Smoke test, thorough visual inspection of all vacuum related hoses, gaskets, spraying carb cleaner around areas associated with vacuum etc.
Some cases a leak will be obvious by the "hissing" sound.

How To Smoke Test For Vacuum Leaks: A Step-by-Step Guide

Learn how to smoke test for vacuum leaks with our comprehensive guide. Find and fix engine problems with this essential diagnostic technique. Get step-by-step instructions and safety tips.

wisepickers.com

This was going to be my suggestion. I was having similar starting issues, and running poorly. It was a vacuum leak between carb and pcv

 

[motionz]

Excuse me but the 400rpm is on LPG en gasoline, sparkplugs are new and can't see already the state of them. In this test test phase I have blocked the vacuumlines of the brakebooster, the pvc (the vacuummeter is connected to pvc hose to carb.) and the both Evap canisters.
The poor running is the same with cold and warm engine, its not realy hot cause I can't drive it.
Compression I can't measure cause I have no tool yet

What was remarkable, however, is that the rear 2 exhaust manifolds (7 and 8) didn't really get warm after running for 5 minutes; I couldn't even touch the other ones anymore!

That was the reason to test the spark plugs, and I noticed that the spark wasn't strong and was sometimes irregular.

Therefore, I wil replaced the ignition coil today (now its raining..) and tested the newly installed HEI module (resistance) to see if it had become defective.

 

[75gmck25]

Buy a package of vacuum caps and start plugging off (take a picture first) the vacuum port on the intake, snd the large and small ports on the carburetor.

Then uncap one single port on the manifold fitting (most are a tree-shaped fitting) and connect your vacuum gauge there. Do your test with spraying brake fluid, or use an unlit propane torch to determine if you can get rpm to rise and find a leaking area. Once you find it, keep replacing gaskets and resealing that area until you get vacuum up to normal.

Did you ever remove and replace the intake manifold with an aftermarket aluminum intake? The aluminum intakes have a thinner flange, and with stock length bolts there can be an issue with the blind holes for the end bolts. The bolts may bottom out too soon and appear to torque correctly, but not really pull the flange down tight. In my experience this leads to an antifreeze leak, but it could also cause other issues.

 

[muldokken]

Well, yep you found one big vacume leak. Probably sucked the gasket in. That throttle shaft on that q jet is pretty worn also.

 

[Rusty Nail]

kinda reminds me of the spade connectors on the dizzy years ago. They were loose and stopped twice on me and had to pull off the road. The 2nd time I figured it out (crimped tighter with my leatherman) and was fully repaired.

Just a thought.

thats a good one.
in my experience them wires and shrink or legnthen with changes in ambient temperature. Sometimes the distributor cap PLUG can fail to seat correctly, especially at that age.
second I am constantly curious about the engine grounds. Mostly at the cylinder head to firewall and battery to fender.
third : you mentioned tossing a bunch of parts around. testing is good ya. but it will not repair or replace the PICK UP COIL at the distributor. Sure you can replace it but it's hard and the parts are NOT easy to get.


In my experience, before tossing ignition parts into a 50 year old distributor, I have found it easier to simply buy a new one - that has all new modern parts in it already.
a "loaded" distributor is your friend, squarebody guy in Europe

good luck


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