Yes, I've searched and read. I guess I'm hoping for a miracle that it's not the opti. I am not one of these people that immediately blames the opti for all the ills of the car. So here's what's happening. Car is a 94 with an automatic. Everything was great until the car ran out of fuel one day and now this issue.

Car backfires under moderate load after getting into high gear. Cruise along at say 45 start to roll into it and it starts popping and back firing. Get onto it heavy from a stop, no issues. Get into it heavy from a cruise instead of moderately, it doesn't like it. Cruise normally like you're just trying to get home, no issues. I can get it to pop sitting still and just blipping the throttle as well. Usually takes about three quick blips to 2.5-3k and it'll pop on the third or fourth.

I've diacomed the thing repeatedly and see nothing. Timing is where it should be, MAP is where it should be, O2s are doing their thing, no knock at all, and everything else reads fine too. No codes. I really wish I could put a wide band O2 on it when it's popping but I don't have an Innovate sitting around.

I've replaced the fuel filter(it was clean). I've pulled the injectors and had them flowed and cleaned. Found an injector stuck open on the bench and thought it was the issue. It started working again fine once it was sonic cleaned but the popping was still there once I got the injectors back in the car. Fuel pressure is right where it should be and the regulator responds quickly and nicely. All the plugs look great and I see no damage to the ceramic. MSD wires are not damaged or burnt. I have not Ohmed them though. Swapped the ICM for another I had on hand, no change. Can't swap coils as the plug's different unfortunately. Checked for vacuum leaks with ether and found nothing. AIR check valves are good. Cleaned the MAF with electrical contact cleaner. No unmetered air is getting into the engine.

I can't think of anything else to check but the opti. The opti isn't that old, I have no codes which I know is not a guarantee, and this all started after the car ran out of fuel.

Anyone have some suggestions?

Thanks

 

My 94 had really bad backfire and it turned out to be a fuel pump but you said that was good. My car was still missing pretty bad and I just changed the plugs to ngk tr-55 with a gap of .040. The old plugs weren't that old and looked good but were gaped at .050. Now car runs wayyyy better with just a slight flutter sometimes.

 

Also running out of fuel is really bad for the fuel pumps, kills them fast. Also when my car was really backfiring another time it was antifreeze getting all over the opti.

 

What were the fuel pressure measurements:
-at idle, vacuum compensation line off
-at idle, vacuum compensation line connected
-at WOT/max RPM

 

idle line off/ pump on engine off ~ 48psi
idle line on ~ 42 psi
WOT ~ 48psi


FSM says these values are acceptable. I was thining they were a little high since 3 bar is ~43.5psi. I tried driving with the line on and off and it made no difference. Possibly a little worse with the line off but I think I may just be getting better at making the problem occur. Oh and the pump is stock/original as is the regulator. When the car is turned off, pressure increases slightly and holds rock steady. No bleed down.

I'm just running regular Bosch platinums gapped to .050"

 

I thought the FSM specified 41-47psi, but I don't own one, so I'll take your word for it.

When you say it "backfires", do you mean it is firing back through the intake manifold, or it is popping in the exhaust?

You actually use Diacom? I haven't seen anyone use that in years. What do the BLM's (long term fuel corrections) look like? They should be below 128, since your injectors are flowing about 5% more than rated, due to the excessive pressure.

 

lol yeah diacom is old but it works. You should see the lap top that I use to run it.

The FSM says that with the pump on and engine off, the PSI should be between 41-47 and it should drop 3-10 PSI once the engine is running. I too thought it was high until I read the FSM making sure I wasn't missing something.

I played with the car some more today. I had a pair of used but known to be good front O2s that I put on the car just to try. Nothing changed. It did however through a code 63 after a while. This causes the PCM to go into open loop. I could still get it to pop even with the SES light on. If I remember correctly, this means the BLMs no longer matter since it's in open loop.

I'm waiting for my laptop to recharge so I can get those BLM numbers as I don't remember them. However, they nor the intergrator numbers raised any flags to me.

I also bought another coil from the Zone since it was only $20 bucks. Still no changes. I tried that and the O2s separately.

This car is not stock. I didn't want any are you sure you did this installation part correctly replies. The car has been modded for years and has had a few thousand miles put on it. It's also been beat on very hard at Road Atlanta. There are no problems with the mods. This problem just started. The injectors are Accel 30#s but they average 27 pounds at 3 bar after their cleaning three days ago now. Car has been dynoed many times with a wide band with the current tune.

Car back fires through the exhaust. The lovely free flowing wide open sounding exhaust gives it a really good high pitched pop like a gun shot when it happens. I'm getting good enough at making it pop repeatedly that I'm afraid I may see people ducking for cover as I drive by.

One thing I have noticed is it really likes to pop between 40-50 mph with about 1/2-2/3 throttle. I can make it pop a couple times around 60 with the same throttle input, both times coming from a cruise throttle not a romp. So I believe it's a load issue as it'll put more of a load on the engine to get the car moving from 40-50 than from say 60-70.

Thanks for all your help.

 

When its in open loop, it continues to use the BLM's it "learned" in closed loop. The only difference is that learning is no longer possible, so the BLM's will not change any more. And the integrators will stay at 128.

 

ah ok.

Well, I think I have a break through but it'll have to wait til tomorrow for verification. When I inspected the plug wires, it was during the day. I waited til night time and pulled the car into the garage and fired it up. Nice little light show was being produced under the hood. I have some boots that need to be replaced it appears. I may also have sparks coming from the opti. I sure hope not though. It's fairly new like I said. So I'll get in there and make the repairs and get back to you.

Also, to answer your questions on the BLMs, they were at 141 and 142 respectively while the popping was occurring. I obviously missed this but the short terms where right at 128 basically while the popping was occurring.

Thanks Injuneer. I'll update this tomorrow.

 

I had a problem like that about 1500 miles after I installed a MSD cap and rotor. It started backfiring and I could see spark scatter at night on the cap andat plugs. I found I was another victim of the rotor screws backing out. I found one screw completly out and it had broke the guide pin off the rotor and egged the rotor screw holes causing it to rock. New cap and rotor and a some red loctite and it runs good again. ( It doesnt backfire or spark scatter anymore). - The thread lock that came on the rotor screws from MSD let me down!
Good Luck,
Lloyd

 

Thanks Lloyd.

Well, I think that was the issue. I had some extra MSD boots around so I put a new one. It pushed right on and it cleared the header primary unlike the burnt one. Guess I didn't put the old one on correctly. All the same, not a hint of back fire. I'll drive it a coupkle more times before I say I'm done. Thanks for everything guys.