Powerful BACKFIRE-- everything looks good cannot figure it out

[Omnivator]

I'm resurrecting my 2.5 TII Petersbuilt powered Omni GLHT and cannot get over a backfire problem. The car ran fine 18 months ago and now I have "fixed it all up" and am experiencing BAD backfire at the boost peak, right after hitting the boost limiter while I am attempting to train the SMEC on the boost limits.

Normally it takes a half a dozen acceleration cycles to train the ECU but I can't get the ECU to learn the ropes this time. Maybe it is the "squishy" heavy duty vacuum/boost line I have for the wastegate instead of my normal hard nylon line but it never did this years ago before I learned about hard vs soft lines so why now.

The SMEC is from FWD performance. The only significant change made that I can think of during the rebuild is changing the wastegate actuator from the small can (originally installed per recommendations from Neil with his custom SMEC) to the large can (sold by Turbos Unleashed and recommended by Cindy for the FWD perf SMECS). It is a high compression (9:1) engine but that is not important?

The timing is good-- started at 9 deg BTDC moved it to 6 deg BTC for trouble shooting,
I checked the cam timing and it is 4 degrees retarded (1/2 tooth),
plugs old but good,
wires good,
wastegate actuator is properly connected and brand new
exhaust is hooked up,-- might have a pin hole 4 feet downstream of the turbo but most likely not...I'll check soon. But a pin hole causing this backfire is not likely.
vacuum at idle about 20 inches Hg
idles great!
No codes except non important 12,22,42
timing light indicates no significant misfire at idle
Fuel in fuel tank is old but ?? 100dB misfire from bad fuel???
Fuel rail holds 50 psi for an hour after shutdown so fuel injectors not leaking much
There is a vacuum leak from the HVAC system but presumably very small (Can't hear it but the heater control system is totally destroyed) so I know there is that one leak in the HVAC system to be repaired after I get this hog running again.....
Fuel pressure gage on fuel rail shows that the fuel pressure regulator is working when I rev the engine in the driveway, It worked fine 18 months ago...

I can't imagine ANY of these minor (non)issues causing this severe problem... I've been playing with these cars for years and have heard a backfire like this before before. The last backfire was so severe I felt it strongly in the floorboards and it left a huge black cloud behind me that appeared to be two lanes wide by 3 car lengths long, it obscured the vehicles behind me until they passed through the cloud.

EGT is around 800 degrees at idle and I have been attempting to get readings at WOT of the EGT and O2 sensor voltage but cannot get the engine controller to stabilize at full boost to get a good reading. It looks like there is a good 0.95 volts on the O2 sensor at WOT at least but that is while I'm feathering the throttle to keep if off the boost limiter. As many of you know the EGT has quite a time delay so under these conditions I cannot get a good reading but there is no indication of running lean.

I looked at the plugs, all OK, have pictures, have not done a compression test but it idles great and exhaust sounds normal as it did 18 months ago. 2.5 inch CAT FREE all the way back to the ultraflow with 3" tip.

Any ideas? At this point I'm dumbfounded and planning to put it back together (after diagnostic disassembly of timing and vacuum systems) and attempt to retrain the SMEC for proper boost limiting. I see no reason why putting it all back together like it was will make any difference so I am ready go give up and trash this OLD POS. I've been working on this "simple rebuild" since September 2, 2012 and cannot devote any more time to this project. TIme to park it back out in the rain and snow an let it rust away for the winter then scrap it in the spring? Very frustrating. I thought I was a top notch TII mechanic but have met my match and cannot spend any more time on this. I have to move on. I am discouraged more than ever.

Lots of good parts in this one... built by a professional engineer with lots of money and time between 1993 and 2000. This is the third Dodge Omni turbo in my learning curve. There are 38000 miles on this awesomely blueprinted customized optimized heavily Peters influenced engine. Full Cage. Custom adj Konies, Coilovers. Positraction. Caster/camber plates. Lots of instrumentation. Strut braces front and rear. This car is built well. It is set up to take 8" rims with 225mm wide tires at 35mm offset. minivan brakes with 7/8" master cylinder. (awesome brakes) I do not take short cuts building mechanical equipment. I cannot believe it but I am stumped and don't know what to do next. I've searched this entire website multiple times for backfire and not a single post comes close to describing what I am experiencing.

This has got to be a simple problem.... What am I missing? I have never heard such violent backfires in all of my life. I warm it up, a little jump in first, floor it in second-bouncing off the boost limiter, third, bouncing off the boost limiter while kind of feathering the throttle to avoid total boost limiting and then POW!! ooooo time to go home.
Thanks in advance for any comments, offers to purchase, or advice.

[glhs727]

whay is your cam timing so retarded and why is the base timing so low? are you havign issues with detonation? and YES! changing compression to 9.0-1 is a big deal....and will change things. but I'm confused, what exactly did you change from when it ran fine for 18 months?

[Omnivator]

The compression has always been 9.0:1 and I have been driving hard and racing hard for years with no problem. I retarded the timing for trouble shooting purposes only. It made no change. I cannot detect any sounds of detonation and the plugs do not display the "speckle of death". I had to drop the engine to change the turbo coolant and oil lines. When I did that I replaced the windage screen and re-did all the perishables, installed urethane filled motor mounts, did a lot of painting, restoration, new Mcleoud 4-puck clutch assembly, new fuel pressure gage, new Westach EGT thermocouple, new Westach oil temperature sender, new instrumentation wiring harness, new Westach water temperature sensor etc, etc, and etc. and also etc.

[Omnivator]

The cam is retarded because it is a half tooth off. I can either go 4.5 degrees advanced or 4.5 degrees retarded. I have been running it 4.5 degrees retarded for 10 years I guess. No problems with this in the past.

[glhs727]

OK, so it's the same motor, injectors, cal etc... as before?

[Omnivator]

Yes. Same ignition system except I had to put in a new (old) distributor because the old one puked as soon as I first fired up the motor. (Then the door handle broke off)
Same plugs, wires, SMEC ... as before.

[Omnivator]

I verified the wastegate is hooked up although if it wasn't then that wouldn't necessarily cause combustion (backfire) in the exhaust system. Verified the wastegate controller solenoid is hooked up although I assume if it wasn't then it would show up in the codes.

Over the last 20 years it seem I have experienced every malfunction possible on a turbododge but never experienced a backfire like this. Nonetheless, i now assume that the cause of the backfire is as follows: When I am attempting to train the ECU of the limits of the engine (after dis- and re-connecting the battery) by performing multiple acceleration cycles, I have been cautious of the inevitable boost limit cutoff and have been tending to feather the throttle when the boost reaches the target. The boost limiter cuts the ignition "ba-ba-ba-ba-ba" and I'm feathering-- maybe the feathering is allowing more fuel to be admitted to the aircharge than normal so after a second or two the exhaust is fuel rich and then POW!. Does that make any sense?

This even sounds stupid to me but ... I'll reassemble the timing and aux belt drives, reset the timing and go attempt multiple acceleration cycles without lifting off the throttle. I expect to hit the boost limiter over and over and over until hopefully the ECU is happy. The risk is that I will blow something up. It doesn't work now so blowing it up doesn't degrade the performance much eh? That would make it easier to "part out" the car eh!!

Other than that I am out of ideas. But maybe my plugs are wore out from sitting 18 months? They were fully functional before so again- I have experienced every malfunction possible on a turbododge but never experienced a backfire like this. OK I'll install new GR5 plugs, properly gapped and then do the restart. (the existing plugs are quite tired so maybe this is the problem)
<(now that I see the pictures it looks like the gap is way way large)>

A big gap might be the problem, I'll let y'all know.
If anybody has any other ideas then I'm all ears! Thankyou GLHS727 for your comments.
Pictures show plug#1, 2, 3 and 4.

[jamierpm]

shit man those plugs are worn out, seems like alot of particles on them, some even seem metallic

[glhs727]

whats your plugs gapped at?

[Omnivator]

'bout a half inch I reckon. Do you think that is too much? Makes for a hot spark. I'll put some new ones in this weekend and let you know how it works out.

[glhs727]

LOL..... well put some new plugs in and gap them at .032 and see what happens. Hopefully it will solve your issue, cheap, quick, and easy

[Glhs60]

I would suggest you run a line directly from the intake to the waste gate to limit your boost to about 8 psi so you dont keep hitting overboost shut down. You're correct in that big exhaust backfires are normally caused by raw fuel in the exhaust igniting when ignition is restored. People used to do this in the old days to scare folks, turn off the ignition while driving, pump the accelerator a few times and turn the ignition back on and BOOM.

Thanks
Randy

[Omnivator]

GLHS60 WHAT AN EXCELLENT IDEA! It may not help program the ECU but it will definitely allow additional trouble shooting without high boost. I should have already put in a grainger valve and I have hard lines running from the engine compartment to the drivers console just for that but ...
Yes, I'm from the old days and intentional backfires were great. One of my favorite backfire machines was an early 70's honda civic. Great fun.
Having trouble sourcing NGK GR5 plugs so will do a search on this website for the latest spark plug ideas. All my turbododge ideas are 10 years old.

[Glhs60]

Quote:

Originally Posted by Omnivator GLHS60 WHAT AN EXCELLENT IDEA! It may not help program the ECU but it will definitely allow additional trouble shooting without high boost. I should have already put in a grainger valve and I have hard lines running from the engine compartment to the drivers console just for that but ... Yes, I'm from the old days and intentional backfires were great. One of my favorite backfire machines was an early 70's honda civic. Great fun. Having trouble sourcing NGK GR5 plugs so will do a search on this website for the latest spark plug ideas. All my turbododge ideas are 10 years old.

I'm a real fan of manual boost. I have a heater fan switch mounted on my shifter controlling 4 solenoids on my firewall. 6-12-18-24 psi at the flick of a switch.

Thanks
Randy

P.S. Thanks for the kind words.

[4 L-bodies]

It sounds to me like you could have a bent exhaust stem which is hanging up when it gets hot (WOT). Mag. bronze guides or iron? Have any idea of clearance of valves to exhaust guides? Your machinist perhaps got exhaust valves guides a bit too tight? But if it ran for years with no problem that is probably not it.