350 Tbi running bad

[74 Shortbed]

If you look at the two pics where he rotated both directions don't look like but couple degrees difference, normal slack, if the chain jumped the rotor would be way off??.

 

[farmerchris]

wonder if a intake manifold gasket leak or a vacuum problem(lines cracked or missing), egr clogged with the backfiring and loss of power

 

[Cuba]

Remember you have to take into consideration initial timing, if it's set at 12* initial timing the rotor will not line up with number one because that would be zero..

It's the base that moves when adjusting degrees or advancing/retarding... so rotor should still point to 1

I guess you could be right about the difference in both pics though. But that would directly connect to slack... still could've jumped a tooth depending on the gears used on this engine. As someone mentioned before, there were engines using a plastic gear for the timing components? Just don't remember which models or years, but quite sure it was a GM thing.

Another thing I did in all my TBI vehicles was to install an inline fuel gauge for checking off things when I do run into problems. I don't think it's a fuel issue here though, just saying.

 

[HotRodPC]

True, not everyone is picky about dizzy placement. And it could be fine the way it is too. I am, though because for me it helps diagnose things a little faster, at least on my vehicles.

I agree with compression testing at least, to check off things of possibility too. It would be nice to avoid having to take front cover off or know for sure if you have to or not.

IDK about the vac test though. When engines are running rough, the needle tends to bounce around too, at least in my experience. Or I'm just not good enough to diagnose with that. *shrugs*

I couldn't tell where the backfires were coming from in the video either, but I believe OP said he'd post another one.

You're exactly right about the vac gauge. BUT, how the vac gauge is acting can tell you what is wrong. We'll learn you something. Here's a link we have as a sticky in the Engine Performance section. If you go to the link included, then look at all the different scenarios of how the vac gauge is acting, you'll see a vac gauge is a very valuable tool for diagnosing engine performance issues.

http://www.gmsquarebody.com/threads/diagnose-engine-problems-w-vac-guage.1570/

 

[HotRodPC]

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Here's another video. It doesn't appear the backfires are coming through the throttle body now. It did it a few times before but now that I've put the cap back on it hasn't done it. Notice I'm the video the stumble in rpms I was steadily increasing the throttle the whole time.


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Just by the sound of that, NO, the chain DID NOT jump time. So moving on... EGR valve would be a good thing to check. Make sure the pintle is not seized and opening and closing fine. Also make sure it's cleaned off, not gunked up with carbon to where it can't seal off completely. I suggest that, cuz it's an Easy cheap thing to do, and you can clean it too while it's out and can be considered general maintenance.

In fact, if you have a vac pump, hook it up to the EGR. Start the motor. Pump up the vac pump. If doesn't change, there's you're problem. If it stumbles badly, and may even shut the engine off, then that's NOT your problem. You're hoping the vac pump to the EGR will cause it to run badly and even shut off in some cases.

 

[Cuba]

You're exactly right about the vac gauge. BUT, how the vac gauge is acting can tell you what is wrong. We'll learn you something. Here's a link we have as a sticky in the Engine Performance section. If you go to the link included, then look at all the different scenarios of how the vac gauge is acting, you'll see a vac gauge is a very valuable tool for diagnosing engine performance issues.

http://www.gmsquarebody.com/threads/diagnose-engine-problems-w-vac-guage.1570/

Nice! Thanks! I assume there's a video in that link but I can't watch on my phone. I appreciate the lesson!

 

[HotRodPC]

Remember you have to take into consideration initial timing, if it's set at 12* initial timing the rotor will not line up with number one because that would be zero..

That's right, and add a little chain slop in there too so it could be off by 1 whole spot. If it's off 2 or more then you can about be certain.

 

[HotRodPC]

Nice! Thanks! I assume there's a video in that link but I can't watch on my phone. I appreciate the lesson!

No, you click on each of the scenarios and then it goes to a video of how a vac gauge is acting then tells you what the problem should be. Not sure if it'll work on a phone or not.

 

[87silveradok20]

Just by the sound of that, NO, the chain DID NOT jump time. So moving on... EGR valve would be a good thing to check. Make sure the pintle is not seized and opening and closing fine. Also make sure it's cleaned off, not gunked up with carbon to where it can't seal off completely. I suggest that, cuz it's an Easy cheap thing to do, and you can clean it too while it's out and can be considered general maintenance.

In fact, if you have a vac pump, hook it up to the EGR. Start the motor. Pump up the vac pump. If doesn't change, there's you're problem. If it stumbles badly, and may even shut the engine off, then that's NOT your problem. You're hoping the vac pump to the EGR will cause it to run badly and even shut off in some cases.

This truck has headers so the part on the bottoms of the engine is removed and just the stuff on the manifold is left, would this still cause a problem?


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[Mr. Gutwrench]

Based on the first video, it sounds like it's back firing through the intake. Check the distributor cap and rotor for carbon tracking.

Is the engine making a ticking noise? Could be a burned valve or rounded cam lobe.

It doesn't sound like a fuel problem, but it wouldn't hurt to check the fuel pressure.

1st follow HotRodPC's advise and use a vacuum gauge. The gauge is cheap and a very helpful diagnostic tool.

 

[smoothandlow84]

To me that sounds like more of a miss than a backfire. Most backfires i have heard are much louder and "crack" from unburnt fuel/timing. I still think its a fueling issue.

 

[4WDKC]

Check the fuel pressure, had an ex-girlfriends car do the same thing,checked everything else first, then fuel pressure it was bouncing all over the place.

 

[87silveradok20]

To me that sounds like more of a miss than a backfire. Most backfires i have heard are much louder and "crack" from unburnt fuel/timing. I still think its a fueling issue.

It's definitely a back fire. It's more pronounced when under load.


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[Cuba]

It's definitely a back fire. It's more pronounced when under load

I agree

You can see the exhaust light up at the end of the clip... although the flash is near the cat versus tailpipe.

 

[1987 GMC Jimmy]

All the aforementioned suggestions are worthy troubleshooting tips. I'd break out the vacuum gauge, Mityvac, and you can rent a fuel pressure gauge from Autozone that should be able to be installed inline where your fuel filter is. I'd check all of those things. If you don't have a Mityvac, you can do the same EGR valve test by pressing the diaphragm, capping your finger over the port, and seeing if it holds. If it's stuck before you can do the test or it doesn't stay in place while you're holding your finger over the nipple, it's a bad valve. Has your SES light given you any clues? Have you made sure that it's working properly?