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Discussion Starter #1
Of course as soon as I buy my LS1 camaro my LT1 starts acting up. They get jealous of each other I swear!

The problem I am having is a very loud single gunshot style back fire only at idle. I have been fiddling with it over the past few weekends but I cant seem to narrow down the problem.

The car is a 96 TA that I rebuilt about 2 years ago. Trick flow heads comp cam. Kind of a cheap street beater

Once the car is in closed loop, the short term on bank 1 rises from 0 to about 50 within a span of 30 seconds. Bank 2 is normal. Keep in mind, this is at idle.

I checked the header gasket, not blown. I noticed that the exhaust port for cly 5 was slightly wet looking but nothing crazy. the other 3 were dry. Although there is a small split on the header on the top of the runner for cyl 5. When I first installed these headers, cyl 3 and 5 developed a split, but at that time I did not have any problems. I eventually welded them closed. I used some JB weld to cover the split that is there now (did not have my welder with me) no change.

The O2 sensors are new within the last 3 months and I can see with the scanner that they are operating properly.

There are no trouble codes.

Any ideas of where to look next? Thank You.
 

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you have a exhaust leek on Bank one, DS of motor if it pegs STFT at 50.

Sounds like the JB Weld and other crack you see in the headers is the reason
 

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Discussion Starter #3
So today I went out and messed with the car some more. I removed the jb weld I had applied and welded the crack in the header closed. I then disconnected the battery and pulled the pcm fuse to reset the computer.

Started the car and it backfired a couple times and then ran smooth. But once again, as the car went into closed loop, the short term on bank 1 began rising and the engine started backfiring again. I noticed that the car will backfire a few times in open loop when the car is first started.

The backfire seems to happen once every 5 seconds or so. And its loud like a gunshot

I turned off the car and unplugged both O2s. Started the engine, still backfires.

I switched the O2s to different banks. Still backfires. Still have rising fuel trim on bank 1.

My question at this point - is there anything other than an exhaust leak that could affect the fuel trim on one bank only?

I want to rule out any other issue before I buy new headers. Which I am guessing are leaking in areas I can't see.
 

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A vacuum leak that affects one side of engine, or exhaust leaks in areas before o2 sensor. Fuel injectors that are clogged and can't spray the proper amount of fuel.

Until you go into closed loop, the pcm doesn't do all the corrections to fuel trims. So, when in open loop your pcm is simply keeping fuel injectors open long enough to spray enough fuel not to run extremely lean.

But the exhaust leak drawing a lot of extra air into engine on one side is your best bet.
 

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you may have to pull the DS header to fully inspect it.

IDK what caused the crack that was JB welded that you then welded...but perhaps there is another crack or broken weld causing a pre 02 leak

If you have long tube headers...sometimes speed bumps will hit them and cause cracks/breaks at tube connection weld points
 

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Discussion Starter #6
Thanks guys. I've pretty much narrowed it down to the headers.

I went out again today and double checked for vacuum leaks by the drivers side. No luck. I also went ahead and re lashed my valves again on that side just to make sure. Ended up finding a cracked polylock nut, which is the 6th time this has happened to me with using these comp cam nuts over the years. Installed a fresh one. But after all this, still having the same problem with bank 1 and back firing.

Going to go ahead and buy some new longtubes.
 

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Discussion Starter #7
I just thought of something.

When running the car without one or both O2s unplugged, It will still backfire in open loop. I can let it idle as long as I want, but if I rev it a little bit, it will backfire once on deceleration.

How can this still be backfiring in open loop like that? If it was bad headers or a vacuum leak, then the car would only back fire in closed loop?
 

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your scan says STFT is 50. That is pegged.....meaning it could be higher.

your intake could be leaking on the DS....do the starter fluid spray test around intake....does RPM suddenly change?
 

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Discussion Starter #9
Yes I tried that trick when I was looking for vacuum leaks with no success. Although it could be broken in the lifter valley which the spray test would not be able to test for. My next plan is to pull the intake and see. But I still don't understand how the car can also backfire in open loop?
 

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If you did the starter fluid intake leak test, don't pull the intake until you can confirm your issue is not something else.
 

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Have you pulled your fuel rail and tested for proper injector spray? I also suggest putting a fuel pressure tester on the rail and seeing what your fuel pressure is.

Should be around 43 psi for key on prime. Mid to high 30's while idling. And around 40 under other driving conditions. Your problem is only on one side of engine, but I've seen all sorts of crazy stuff happen with these cars.

If you happen to have a spare known good pcm, swap it in and see if it makes a difference. Pcm rarely fails, but it is 22 years old. This would eliminate pcm as a suspect.

While you have the scanner on car, have you checked to see what the o2 sensors are reading? Bank 1 sensor 1 & bank 2 sensor 1 are the important one's. They should be constantly changing above & below the 450mv center reading.

If you don't have the 96 service manual get it at My Files

Starting on page 6E-470 is the pcm conector chart with symptoms listed if wiring, connectors, or pcm signal if faulty on that cavity of pcm connector. Might want to study whole drivibily & emission section of manual. Wiring diagrams start in section 8.
 

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Discussion Starter #12 (Edited)
I've been working on the car. Here is a video I made of the problem happening live -

New parts/things I’ve tried since problem started:
Exhaust Header gasket
Fuel injector o rings and i also swapped banks of injectors to see if the problem would follow to bank 2 - no luck
Crankshaft position sensor
Maf and pigtail
Intake manifold gasket
Grommet on side of intake manifold
Checked for vacuum leaks
Checked spark plugs
The o2 sensors are newer. I tried switching them between banks but the rising fuel trim problem stays with Bank 1
Checked pushrods
Checked for broken poly locks
Re-adjusted valves
Checked plug wires visually

I can get the engine to stop backfiring by unplugging and plugging back in an injector to get the fuel trims to 0 out. Once the computer tries adjusting the mixture, it starts backfiring.

Coco - Unfortunately my computer won't let me download the manual :( The 1 O2 sensor is behaving somewhat normal. But one of them stays around 170. I tried switching the o2s and it stayed the same
 

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Discussion Starter #13
By the way, that aem wideband has always read full lean like that even when the car wasnt backfiring. It would also stay like that while cruising but when i would get deep into the pedal it would go like 12.7
 

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An o2 sensor that is stuck in 1 place is not a good thing. Since your obd 2 car has 4 O2 sensors, take readings on all of them. I get confused as to which ones are before the cat and which ones are after the cat..

All O2 sensors should be activly changing readings. But the before the cat O2's should be changing the most. The after cat O2's change readings but not as dramaticly as the pre cat O2's.

But I'll say it again, no O2 should be stuck on 1 reading. Either O2 itself is bad, It's heater not working, or wiring & connector problems. It's common for wiring to touch hot exhaust pipe and burn insulation and shorting out O2.
 

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Discussion Starter #15
I was messing with the car today. I think I should clarify some things. The car only has upstream o2s. The cats are deleted and the downstreams have been tuned out.

So today I went and soldered in a new drivers side o2 pigtail on bank 1. Fired up the car and noticed that bank 2 was stuck on 1.060. I went and installed 2 fresh o2 sensors. Finally got the o2s to fluctuate normally but the car still back fires.

If I unplug both O2s and the car goes into open loop, the car will stop back firing and run normally.

BUT the car will backfire during that few minutes the car is in open loop when its first started. (weird)

I had a spare PCM that I swapped in. Car still backfires.

How would I test for injector spray when they shoot out of the rail on prime?
 

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Discussion Starter #16
Just checked fuel pressure. 45 on prime then pressure bleeds off. These TFS 30lbers want 43.5 psi so thats what it is while at idle and under load.

I noticed now that both banks are trying to run rich
 

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Discussion Starter #17
I replaced the drivers side header today with a brand new one to make sure my problem is not an air leak. NO CHANGE :mad:

My question is this - What would cause both banks to increase the fuel trim so much that it backfires like it does in the video that I provided? Other than vacuum leaks, bad PCM, bad fuel pressure, bad o2s, bad MAF, bad crankshaft position sensor, etc. (all the things I've mentioned).

ALSO, why will the car cure itself with the o2s disconnected? WHAT IS CAUSING THIS THING TO RUN RICH!?!?!
 

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When fuel trims go up, higher than 128 or 0 depending on how scanner displays data, it means that pcm is seeing lean running and adding fuel. If mixture is rich fuel trim will be lower than 128 or 0 and pcm will give less fuel.

Car can be running pig rich and not backfire. It's too lean a mixture that causes backfire.

When car is cold or o2 sensors disconnected, it's running in open loop mode. In this mode the pcm fuels the car by a set of tables and relies on map, & coolant temp sensors to adjust fuel mixture. Engine tends to run richer.

When engine warms up pcm goes into closed loop operation. During this time the pcm uses the same sensors as open loop operation plus maf, o2's, opti signal, & others to determine fuel mixture.

The fact that in closed loop operation that pcm is adding as much fuel as it can and your backfiring, indicates a very lean fuel mixture. You need to put car back on scanner and do a scan with o2 sensors unplugged. With car not backfiring. Have an assistant drive car while you monitor data.This will be your base scan.

Then scan car with o2's plugged in and warmed up. Have assistant drive car again and you monitor data. I'm hoping your scanner or scan program can save data for comparison. See which sensors are not working correctly.

If sensors are working within normal ranges, then a vacuum leak somewhere is the 1st suspect. Your engine could be getting unmetered air into the cylinders. The pcm reads the amount of air entering the maf and adjusts mixture accordingly. Extra air coming into engine is not accounted for. The o2 sensors see mixture as lean and adds more fuel. But pcm can only add so much fuel.

Mixture is still lean with backfiring.

The only other suspect I can think of would be a glitch in the optical part of the opti. However to check this theory you would need an oscilloscope to check low & high resolution pulses sent by opti to pcm. You can buy a pc based scope on Ebay, Amazon, etc for about $70.

This is a tool that I recommend to all lt1 owners.

One more thin, have you tried disconnecting the cat and running without it to check for clogged cat?
 

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Discussion Starter #19
Coco, Thank you so much for all the info. I feel like an idiot; I had not once thought that the car could backfire in a lean condition. Makes sense though, extra air in the exhaust equals extra O2 which we all know is explosive with the right catalyst.

My scanner will do live data recording and playback so I will have to do what you suggested. Although the car is not legal right now. No plates and insurance. Everything I've done has been with the car sitting in the driveway.

As far as the cat, I have none on the car. Car is ""track only""
 
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