90 gmc k1500 5.7 tbi backfire problems

[Pinger]

Don't know if this will help.... but I'm running a Vortec 350 on LPG/propane and am currently re calibrating the fueling to eradicate an inlet backfire which can be induced with a snap throttle opening from idle.

My problem lies in me disconnecting the PCV inlet to the passenger valve cover from the inlet tract and allowing it to breathe un-metered air. At idle, the driver side PCV restricts air into the manifold. When the throttle is opened so is the driver side PCV which momentarily weakens the mixture hence the tendency to backfire if I go into the throttle faster than the LPG electronic control can react.

A (momentarily) lean mixture causes it and un-metered air through the PCV system is the cause of that. Is your passenger side PCV connected as it should be?

 

[PlayingWithTBI]

A (momentarily) lean mixture causes it and un-metered air through the PCV system is the cause of that.

That's where AE (Accelerator Enrichment) comes to play. It's like a pump shot in a carburetor. Unfortunately I don't think your LPG system has that?

 

[Erik the Awful]

You're running a bigger cam, but the truck was running fine before you pulled the distributor and oil pump? Ignore the fuel map issues for now - you do want to fix that, but after you figure out what's wrong with your ignition.

Recheck your wiring order. Pull your driver's side valve cover and ensure your 0* mark matches up to TDC #1. Check the condition of your cap and rotor.

Edit: Let me clarify that I don't think checking the fuel map is wrong, but I think you should recheck the ignition system carefully before going down that road. Your fuel map is absolutely a contributor to the problem, but it'll be a snipe hunt if your ignition isn't sorted first.

 

[Pinger]

That's where AE (Accelerator Enrichment) comes to play. It's like a pump shot in a carburetor. Unfortunately I don't think your LPG system has that?

You're right - my LPG system can't do that. Instead, it has to be set up to avoid it ie, mechanically lean (via the regulator settings) and via the same settings, rich at throttle opening. That way the electronic correction is moving towards lean and it's tardiness doesn't matter.

Interesting that despite all the air being metered via the MAF sensor (even the air passing through the crankcase) the injector systems still give an extra shot of fuel as per a carb set up. I'm guessing that's a TBI thing (to counter fuel drop-out) and that the sequential system on the Vortec doesn't need it?

 

[PlayingWithTBI]

Your fuel map is absolutely a contributor to the problem, but it'll be a snipe hunt if your ignition isn't sorted first.

Agreed, it's almost impossible to tune out a hard part issue.

 

[Darkelden]

i guess i am not understanding, i have tried a bosch scanning tool and it is just a aldl ob1 plug, i have just also tried a snapon solus scanner as well and it does the same thing with the rpm's. i have tried to adjust it based on idle from the machine in comparison to the idle it shows on the tack for a ruff go at it. i cant seen to get past the ability to set the base idle correctly on this truck for the life of me. in the last screen shot you sent me,is that the idle i should set it to when the scanner is hooked to the truck?
I also tried to search online for the prom/ecm i have for the base idle table. it shows prom is 16139461 and 1227747 for ecm. each page i look at shows different base idle info, one showed 750, one showed 600 rpms, I guess i am having troubles getting correct data via google to set base idle along with in-proper training on how to use the equipment.

the only difference i have seen when i switched to the snapon scanner it now shows knock sensor data. When i was adjusting the base idle last time it showed 77 registered knocks.

I went over the whole engine again, verified that there was no vacuum leaks,

checked vacuum on each component and all were within the normal rated range.

verified the spark plug wires all had good connection and are in order and were gapped correct at .35

I feel like i am missing something simple on my end that i keep over looking or just keep doing it wrong.

 

[studigggs]

On the Solus you have to select “road test data” in order to bypass diagnostic mode (when rpm sets to 1,000 rpm). You’ll generally register some knock counts at startup just from the shock of the starter engaging and disengaging But as long as the ESC Counts dont keep adding while running it’s normal.

 

[PlayingWithTBI]

I also tried to search online for the prom/ecm i have for the base idle table. it shows prom is 16139461 and 1227747 for ecm.

The PROM ID is a GM designation that doesn't mean much of anything to most of us. In order to get your 4-letter BCC (in this case it's AKCW) you can either look on the ECM or open it up and look at the chip. Yours will look like the one on the right in this pic.

You must be registered for see images attach

in the last screen shot you sent me,is that the idle i should set it to when the scanner is hooked to the truck?

The screen shot I posted is from a 7747, 5.7L, and 700R4 trans AMUS .bin. So, you'd set your base idle 125RPM less or 475RPM , maybe a little low for practical application. I have mine idling at 650 w/o A/C and 725 w A/C as an example. In essence, you just need to set it below target speed so it doesn't conflict with the ECM's control of the IAC. If you figure out what BCC you'll have a better idea what's going on.

Again, as @Erik the Awful said, something changed when you replaced the oil pump, you need to figure that out (if it ran fine before).

 

[Darkelden]

i will take out the computer tomorrow to look at the 4 letter code to verify. i just check all vacuum lines again just to make sure no leaks at all. so far the map vac was at 19 hg and with breather needle open it was at 16 hg , the pvc was at 16 hg with breather needle open and 16-19 with rapid fluctuation when the needle was closed.
The egr side on tbi was at 15 hg with breather needle open and 15-17 hg with it closed
between egr solenoid and egr there was no vac at all.

When i replace the oil pump in the truck, all i did was unbolt the engine mounts, removed all wires that were to short to allow me to lift the engine up alittle to slide the pan out. no other modification were made. After it was all back togeather i ran the truck and the idle was really low around 300-450 ish. that is when i first tried to reset my base line idle, i put a connecter between pins a-b for 30 seconds with the key on, then went and un plugged the the iac connector. after that i then started the truck and adjusted the t20 screw and set the idle at that time to 750 via the after market tac. turned off the truck and then plugged the iac back in. i tried to adjust my tpc, but the information i was proved said to set it to .68 via online form. the form that i have provided in this group has shown that it was way wrong. that is needed to be at .58-.59, i just put a new tpc in today( with no expanded wholes) so set it back to factory ish area. at this time it shows the tps is set at .72. i dont want to drill it out just yet before i can set base idle correct so i dont waste more money.

i am still reaching out the tech who helped me build the engine to find out the exact cam we put in. if i remeber it was a rv cam ( something like this ish
Chevy SB 283 305 327 350 400 RV/TORQUE Camshaft/Cam+Lifters CL Kit .421/.444 (Valve Lift .421"/.444"))


i was also told i might need to take the timing cover off again just to verify it did not some how jump tooth or is set 180, i was told that should have been noticeable when i did my compression test on each cylinder but they said at this point it doesn't hurt to look at it.

 

[Darkelden]

On the Solus you have to select “road test data” in order to bypass diagnostic mode (when rpm sets to 1,000 rpm). You’ll generally register some knock counts at startup just from the shock of the starter engaging and disengaging But as long as the ESC Counts dont keep adding while running it’s normal.

when i would hit the gas it would keep counting after startup. usually it would add 1-3 knocks each time bring the engine to 2000 rom