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Manifold backfire & hesitation?

Hossbaby50

Hossbaby50

3/4 ton status
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Sep 1, 2001
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Location
Peoria, AZ
I am currently trying to diagnose a problem with why my truck is running poorly. It is backfiring through the manifold and has a hesitation off idle and through about 2000 rpm. The backfire only occurs under heavy throttle in park and when it is under load and you try and go WOT.

Background. 85K5 w/MAF TPI setup. Heated O2 sensor, no VSS. The truck believes it is in park all the time. It has run perfect in the past, this problem is a newer problem. There are no stored codes.

I have changed ECM's, EST's, ignition module, all relays, IAC is pretty new, TPS seems to be reading fine. I have reset the IAC and adjusted minimum air. I just cleaned the plugs and they looked ok. Timing was a 0* and I have bumped that to 6* to see if it helped but it doesn't seemed to have done much or anything. I have disconnected the MAF to see if that helps but it doesn't do anything beside set the code light. I have checked vacuum lines and they seem to be ok for leaks and such. PVC is ok.

I am to the point I am thinking about ripping off the manifold and redoing the gaskets on the TB and the runners.

Any ideas or help?

Harley
 
Manifold being exhaust or intake?

I assume you mean exhaust, but if do mean on the intake side, that's likely an ignition issue.

'85 is the one with the weird ECM setup isn't it, something on the timing side. I know it's the one people say to avoid at all costs due to the one-off electronics for that year.
 
My bad.

Intake manifold backfire & '87 MAF TPI from a Firebird. Small cap HEI with remote coil.
 
Hesitation from idle up could be caused by low fuel pressure. After checking that put a vacuum gauge on and look for a steady reading, any fluctuation indicates a intake valve going bad which will cause the popping in the intake. Then compression test.
 
The fuel pressure was checked not to long ago and was solid at about 41 psi at the rail.

After running a can of Throttle body cleaner through the intake, cleaning the MAF, and reinstalling the heated O2 sensor (had a non-heated previously) it runs better but still has some of the hesitation and will pop through the intake.

Harley
 
No, but i have the same problem with either one of my pumps, the main or the spare. I will check it again this weekend with the line the regulator disconnected.

Harley
 
Reason I asked is because the FPR is vaccum referenced. When you get into the throttle vac drops and the FPR boosts pressure. Your problems sound like you may be going lean under acceleration/ WOT. I think you should be around 43 PSI steady with the Vac line disconnected.
 
I had the same thing and mine turned out to be an over tight intake valve adjustment.... so if you adjusted the valves recently check that... or maybe do a compresion test make sure you dont have a burnt or stuck valve.
Just an idea!
 
I am getting to that point right now. I am going to reseal the intake plenum and EGR, replace the intake air temp sensor, and replace the coolant temp sensor. If that doesn't do anything then I will check for compression/valve issues.

Harley
 
Hossbaby50 said:
I am getting to that point right now. I am going to reseal the intake plenum and EGR, replace the intake air temp sensor, and replace the coolant temp sensor. If that doesn't do anything then I will check for compression/valve issues.

Harley
Click to expand...
Check for a vacuum leak first, no reason to do work not needed. Also check EGR. IAT and CTS sensors hardly go bad and can be checked with a scantool or WinALDL from a laptop. Compresion test is easy too!
 
I think my problem may be related to a vacuum leak but I can't find it with the old method of running propane along the gaskets on the plenum and TB. When I pick up my gasket set from Checker tomorrow I will rent a compression tester.

Harley
 
I've never had luck with propane either. But this method posted below always finds the vacuum leaks.

eagle mark said:
This is dangerous but works well, do it outside, no wind. Spray short small bursts of starter fluid to specific spots from about 2-3 feet away. If the motor revs, you found a leak. Check around TBI unit, intake manifold etc...

Please be careful, let the previous spots dry before doing another, don't spray near alternator. It takes me less then two minutes to check an entire engine and I have never had one blow up...:eek1: it'll just be a flame up real quick like when you seat a bead of a tire on a wheel...
Click to expand...
 
I tested compression tonight. All cylinders are between 140-150 PSI. It is idling better now that I have the heated O2 sensor back in and have fiddled with stuff, but I still get the pops under load and stumble when you punch it off idle. The truck doesn't seem to have as much power as it should either.

It will pop/hesitate with and without the MAF hooked up.

I tried finding any vacuum leaks with brake cleaner and I may have found one near the passenger side runner at the manifold or the EGR valve. I am tempted to remove and block off the EGR valve all together since my motor doesn't ever use it since it doesn't have a VSS.

I got the gaskets to reseal the intake plenum, runners, and EGR. I will probably install them tomorrow night.

Harley
 
Yep. I had the timing set at 0* originally, and I have since set it to 6* per some posts I have read on Thirdgen.org. I am planning to put the timing back down to 0* tonight just for grins.

Harley
 
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