'98 Tahoe 5.7 Vortec runs like crap, backfires in exhaust and intake - need help, pls

[CrustyJunker]

I don't think spark plug gap would make it run THAT bad. It's not impossible, but I wouldn't think it's likely. I also have my plugs gapped at .040 and I have never had a problem in the 13 years, three sets I've put in, or 170,000 miles I've put on having them set up like that.

Lifters on these are always noisy on cold start...Typical small block Chevy. As long as you got oil in it and it quiets down within several seconds, it's no big deal.

Once I had a poor quality aftermarket fuel pump that was weak but didn't completely fail. It wasn't putting out the volume to keep up and wouldn't go over 3,000 RPM under load but would be able to hit 5,000 all day in neutral. Showed fairly okay fuel pressure at idle and revving it unloaded (~50 PSI). It didn't show any check engine lights or anything until one time cruise control made it go lean and misfire for too long. Then I got low voltage O2 codes. That could be something to consider. BUT...

What's got me thinking is your poor idle. That almost sounds ignition/timing related. When I had a bad distributor gear (that rides on the back of the cam), I had a check engine light on it -cam/crankshaft correlation fault. This caused poor and rough idle, and low RPM cruising it chugged and sputtered with an occasional backfire. How many kilometers do you have on your truck, and has that gear ever been changed?

 

[1997chevydriver]

I still think that plug gap isn't right. Your old plugs looked to be at gapped bigger than the new ones.
I agree with heavychevy, check your spark plug wires for proper routing, make sure ya didn't cross a couple wires.\

And Paul, he replaced plugs, wires, dizzy cap & rotor.

Didn't see it in his OP the first time i read thru.... Whoops

 

[Tempritscher]

Hi.

Yes, changed all spark plug wires, cap and rotor. The wires are properly installed on the right cylinder, yes.

I've got Chilton's GM Full Size Trucks 88-98 Repair Manual.

The truck is currently in a garage, where the mechanic should search for the problem and also change the leaking intake manifold gasket.
Searched myself for about two or three weeks after work and no success. I need my truck, it's my only car...

I could also imagine bad ignition timing, the mechanic said sparks came outta every spark plug so far.

Ive got 208,000 km / 129,000 mi on my truck. Got it in Fall 2012 and don't know, if the dizzy gear ever has been replaced.
But the mechanic will see that gear condition. When he replaces intake manifold gasket, he has to remove the distributor.

I pleased him, that he should mark and turn the dizzy a bit while engine runs to test if it runs better, before he unmounts the intake.

But I never got a cam/crankshaft position sensor fault or so from OBD.

 

[phule]

What codes are you pulling?

Adam

 

[Tempritscher]

I'm getting no codes at all.

Sometimes the MIL was flashing, when misfiring in exhaust.

 

[CrustyJunker]

If you never got the MIL for a cam/crank fault, your distributor gear is probably okay. If you're more than 2° from TDC while cranking, the truck won't start. 129,000 miles is really not high enough miles for strange problems like that.

I know it's not easy, but try to check your fuel pressure under load if you can. MIL flashing is usually excessive knock and could be a possible lean condition. If it's not a vacuum leak, and you ruled out many of the common sensors already...Fuel pressure is the only other that I know of that can cause an issue like that without an MIL.

Good luck.

 

[1997chevydriver]

If you never got the MIL for a cam/crank fault, your distributor gear is probably okay. If you're more than 2° from TDC while cranking, the truck won't start. 129,000 miles is really not high enough miles for strange problems like that.

I know it's not easy, but try to check your fuel pressure under load if you can. MIL flashing is usually excessive knock and could be a possible lean condition. If it's not a vacuum leak, and you ruled out many of the common sensors already...Fuel pressure is the only other that I know of that can cause an issue like that without an MIL.

Good luck.

Thats false. The dizzy can be more than 2 degrees off. The dtc for cam to crank correlation dont set till 15 degrees off. Also a flashing SES light is usually multiple random missfires

 

[GMstrong]

get scanner hold the motor at 2k rpm and it should tell you the exact cylinders that are misfiring Im betting its the dizzy gear that rides on the camshaft they wear out quick replaced one on my old 2500 was ready for a new one within 400 miles.Bought 5.3L ls problem solved!

 

[Tempritscher]

Seems to be a defective dizzy gear. The mechanic moved the dizzy and engine runs definitely better now.
Today he will have to take out the distributor because of replacing the leaking intake manifold gasket.

Anyone knows which gear is the right for my 98 Tahoe 4WD 4 spd Auto?
Maybe ACCEL Series 37? http://www.usparts-shop.de/zuendsys...inger-und-module/8727/verteilerritzel?c=21242
Maybe ACCEL 34+41 Series? http://www.usparts-shop.de/zuendsys...inger-und-module/8730/verteilerritzel?c=21242

 

[kamokevin]

Seems to be a defective dizzy gear. The mechanic moved the dizzy and engine runs definitely better now.
Today he will have to take out the distributor because of replacing the leaking intake manifold gasket.

Anyone knows which gear is the right for my 98 Tahoe 4WD 4 spd Auto?
Maybe ACCEL Series 37? http://www.usparts-shop.de/zuendsys...inger-und-module/8727/verteilerritzel?c=21242
Maybe ACCEL 34+41 Series? http://www.usparts-shop.de/zuendsys...inger-und-module/8730/verteilerritzel?c=21242

This is the gear that rockauto suggests. GM part #10457356

http://www.amazon.com/ACDelco-10457356-Distributor-Shaft-Gear/dp/B000C9BXDC

http://www.rockauto.com/catalog/moreinfo.php?pk=96139&cc=1303872