1994 5.7 TBI running rough, backfiring

[Fleetmon]

1994 1500 5.7 TBI (3 year old Jasper engine - 20k miles).

Background;
Grandson drove truck 75 miles to my place backfiring and barely running. Truck had been overheated several months prior and I assume it was HOT. CTS was not grounded and apparently fuel-washed the cylinders...several shots of oil in cylinders brought compression back except for #1. Removed head...machine shop found valves warped badly:::bought two reman heads and installed.

Today;
put back together, checked cam timimg and set ignition timing to 0. Started and ran a little rough but not bad. Set timing with light after disconnecting wire at dash and again timed to 0. The truck was running perfectly in the garage for 35 minutes, revving and idling fine...pulled it out to do a road test and revved a few times and blam, backfiring, won't idle. Truck was at a slight downhill angle so I'm wondering now if I'm also dealing with bad fuel. Attempts at starting now result in absolutely no power when it does start, won't respond to accel, backfiring. If I keep playing with it it seems like cylinders are coming back on-line and then dropping out again. Both cylinder heads were a mess though with numerous warped/bent valves. The m/shop did not have a favorable attitude towards Jasper.

Any ideas folks? I'm really frustrated at this point especially since it was running two hours ago perfectly. Compression after head replacement is at 140.. Cylinder walls show good cross-hatch.

 

[morepwr]

When you say backfire, is it threw the exhaust or the intake?
Its not real easy to check fuel pressure on these trucks but they are very sensitive to it not being right. The regulator is on the back of the TBI unit and can be reached by taking the torx head screws of the top of the TBI unit. There is a small diaphram inside it that can split and then it is unregulated.
What do the plugs look like now that it started doing this?

 

[ms21043]

How's the ignition system? Had a similar problem on a 93 S10 truck with the 4.3 V6. The pickup coil in the distributor was bad and would act up after the truck had been running for a while due to heat soak.

 

[Phishin]

TBI rebuild kit is cheap and easy to do. Did it on my truck in a few hours, taking my time and thoroughly cleaning everything. Sounds like fuel problem of some sort. Fuel pressure should be around 13 psi if you can check it.

How is spray pattern on to of butterflies when at idle? Should be nice and round, no drips.

Coil hardly ever goes bad on these trucks. The ignition control module under dizzy cap does tho. When you have this checked at Autozone or somewhere similar, inspect rotor and cap.

 

[mechtech2]

Ignition timing sounds retarded. 10 sounds a lot better than zero at idle.
Give her a twist and she'll probably be just fine.

 

[Phishin]

Originally Posted By: mechtech2
Ignition timing sounds retarded. 10 sounds a lot better than zero at idle.
Give her a twist and she'll probably be just fine.


No....if you unplug the est wire, you set the base timing to zero. My wire disconnect is against the firewall under the black box where the fuel pump solenoid is. I think the wire is purple on my 1990.

if the truck is still running with timing at 0, it advances once you plug the est wire back together.
The timing is advanced according to a set table and a few variables/inputs.
Change fuel filter first. Rebuild TBI if spray is irregular or dripping. If fuel pressure is low still after fresh fuel filter...you will need new fuel pump.....temporary fix is to put a quarter....25 cent piece, under the fuel pressure regulator spring, which will increase fuel pressure until the pump eventually dies.

 

[OVERKILL]

That's interesting. Most distributor-equipped engines I've timed all require a base amount of advance. For Ford engines, it has been 10 degrees. IIRC, it was the same, or very similar for GM ones.

 

[Fleetmon]

Plugs are wet when acting up but they are firing. Acts like way too much fuel but no smoke: Backfiring through intake. Fuel "cones" look good and no drips, however, it appears to be injecting a lot of fuel...almost continuous?

To date: new MAF, CTS, dist cap, plugs & wires, ign module (under dist cap). This prior to replacing both cyl heads.

I think I will play with the timing a bit to see if any improvement. It acts like it has a clogged exhaust, bad fuel, flooding.....all at the same time??

 

[Fleetmon]

the ignition does advance when I re-connect the wire....I noticed while running and the wire connected, it came back to zero...right about the time the EGR opened (by sound to me). If I rev it, it advances hovers then is a hair off of zero.

It seemed to run ok with the exhaust unhooked at the cat today although too loud in the garage to diagnose anything. Cylinder temps are running around 400-500 F at the manifold outlets....all are pretty close to each other and side to side.

Again folks, I appreciate your help!

 

[OVERKILL]

Originally Posted By: Fleetmon
the ignition does advance when I re-connect the wire....I noticed while running and the wire connected, it came back to zero...right about the time the EGR opened (by sound to me). If I rev it, it advances hovers then is a hair off of zero.

It seemed to run ok with the exhaust unhooked at the cat today although too loud in the garage to diagnose anything. Cylinder temps are running around 400-500 F at the manifold outlets....all are pretty close to each other and side to side.

Again folks, I appreciate your help!


It should never be at zero, that's WAY too little advance. Minimum should be ~10 degrees advanced. So if you are seeing ~0 degrees with it running, how the timing has been set is wrong for this particular application, or there is something wrong with the unit controlling your timing.

Either way, I would try with your base set to 10 degrees and see how it works.

 

[Fleetmon]

I'll give the timing a try tomorrow first thing. If no improvement I think I'll also disconnect the exhaust behind the cat....it is a pretty new exhaust system and it looks like it may be a JC Whitney kit or similar.?

It certainly acts like a clogged exhaust the more I think about it....hmmm....warped/burnt valves could be caused by excessive back pressure.

 

[morepwr]

Originally Posted By: Fleetmon
the ignition does advance when I re-connect the wire....I noticed while running and the wire connected, it came back to zero...right about the time the EGR opened (by sound to me). If I rev it, it advances hovers then is a hair off of zero.

It seemed to run ok with the exhaust unhooked at the cat today although too loud in the garage to diagnose anything. Cylinder temps are running around 400-500 F at the manifold outlets....all are pretty close to each other and side to side.

Again folks, I appreciate your help!


Did you shut the engine off and then restart it after you set the timing and hooked the wire by the firewall back up? If you didnt then it doesnt reset the ECU so your timing was going to stay at zero.

Even though its loud, if you can drive it around the block or something while you have the exhaust disconnected may tell you if the cat or muffler is plugged up.

 

[Phishin]

Timing should be way advanced at idle with the est wire hooked up. It shouldn't be at zero. Not really sure where it's should be at specifically at idle, but a few inches left (toward the passenger side) at idle. Open the throttle and the timing mark should quickly wander further left (further advanced).

Maybe your ECM is bad. Maybe your timing chain skipped a tooth or wasn't properly installed when they put the new heads on.

My truck burns a lot of oil. Like a quart every 400 miles. It has done so for awhile. More than 20k miles. I recently removed my CAT, and it was partially plugged, but not enough to effect engine performance. FWIW. Maybe yours is plugged.

Sounds like you have enough gas. I'm starting to think it's a timing problem. Please keep us updated. I've worked a ton on my truck, PM me if you want my cell number, and we can chat while you're in the garage.

 

[jegs]

My son was having simlar issues with his Chevy TBI pickup. It was the distributor cap. I see in your list of new parts you have a cap listed but no new rotor. Make sure the cap is still good and clean on the inside and maybe try a new rotor if unhooking the exhaust doesn't fix it.

 

[Fleetmon]

I may have to give up on this one....new fuel filter and TBI rebuild and the same thing....very rough running, backfiring badly, and absolutely no power....truck bogs down and stalls when put into gear.

When I time it by ear, it idles perfectly and revs although sometimes it will stall at idle. The problem is it won't start at this setting....like timing is way off. What controls the timimg on this truck...ECU?

This thing has become impossibly frustrating....any help is appreciated.

 

[OVERKILL]

Originally Posted By: Fleetmon
I may have to give up on this one....new fuel filter and TBI rebuild and the same thing....very rough running, backfiring badly, and absolutely no power....truck bogs down and stalls when put into gear.

When I time it by ear, it idles perfectly and revs although sometimes it will stall at idle. The problem is it won't start at this setting....like timing is way off. What controls the timimg on this truck...ECU?

This thing has become impossibly frustrating....any help is appreciated.


Did you try setting it to 10 initial? (instead of by ear)

 

[Fleetmon]

Phishin, you bring up something that caught my eye.....when timing with the wire disconnected it is at zero....when wire is connected, the timing mark goes towards the drivers side, not passenger side.....what's up with that?

 

[SteveSRT8]

If the rest of the engine is correct (and this sounds like a spark issue, timing and or firing order stuff) then your exhaust is plugged. Do you have a cat on that thing?

Pull the exhaust off and run it open for a minute.

 

[Fleetmon]

OVERKILL- I cannot get it to advance that much....are you talking about static timing at 10 btdc?

 

[OVERKILL]

Originally Posted By: Fleetmon
OVERKILL- I cannot get it to advance that much....are you talking about static timing at 10 btdc?


Yes, set the base timing to 10 degrees btdc, which should be a very slight twist of the distributor.

If you are able to get it to not start timing it "by ear", you are likely well past 30 initial FWIW.