texas97z

Not car in signiture here are specs:

SCAT 9000 383 crank, forged i beamrods, forged 11:1 pistons, XR 288 grind on LT1 cam core, stock cumbustion chamber cc's, ported heads/intake, new lifters/pushrods, new ford red top injectors, new oreilly adjustable fpr, long tube pace setter,borla cat back, no cats, 58 mm, electric water pump, 6a-L ignition box,auto, tuned by pcmforless, 97 lt1 f body. both front 02's are new as is opti.

Rear 02's deleted, crank position delete, and tuned with all above info.

Car ran great for three days. Then backfire started. Plugs and wires looked fine but we replaced with OTVC taylor wires and NGK plugs. Back fire got better(not as often) but car is still missing and backfire. no regularity to backfire. Fuel pressure regulator seem to be bad as with set screw backed out as low as it gets is 45psi at idle with WOT at 50psi. Back fire usually occurs, if it will, when quick accelration is made. If ease into it, no backfire usually. Is running PIG rich. Seems to be getting good spark to all cylinders.

Please share thoughts as we have been trying to track this down for a while and no fix.

LearJet

The fpr needs to be addressed before you can start tracing this down. a stock one from the auto parts store should do you just fine.

Guest47904

Not too sure about a 383 with stock rate injectors. I would expect the flow rate to be at least in the 30's.

Usually throttle movement like your talking about is a bad TPS. How old is yours?

texas97z

I am getting a replacement fpr today. I agree is first step. Still concerned that the fuel pump that may have been installed prior to purchase was wrong as 50 PSI at idle is way high.

Speedy- The injectors are Ford 30 lbs, and the TPS is original to the car with less than 60k on odometer. I guess age plays as much a factor in the question but before the engine build and larger injectors there was a hard start condition but no back fire/miss fire.

I am not sure what fuel pump is in that tank but it is reading about 5-10 psi too high at fuel rail. Am I too far off by thinking fuel pump has too much pressure for regulator, blew it open and is causing pig rich condition= miss fire/back fire due to unspent fuel in exhaust?

Thought- If I use an after market rail mount adjustable FPR like aeromotive or holley will it be able to lower the fuel pressure to needed amount since pump is making 7 PSI more than stock apparently and blowing past stock type FPR? The range says 30-70 PSI but I was thinking you will not be able to go LOWER than pressure at fuel pump? This might be best fix if that will work.

Guest47904

That's funny cause you never mentioned it but your signature says" 97 Z28 M6 hardtop, daily driver,95 lt1 pace setter mid length headers and x-pipe,24 lb injectors, 383 with barely ported stock castings with a 3-angle valve job,self ported intake to match,1.6 rr,GM847 (stock cam re-grind), 52mm TB, de-screened Mass,!AIR,EGR!380 rwhp and 395 rwtq"

Maybe if you gave us information that we didn't have to dig out, you could have better responses.

I also doubt 50lbs is high enough to cause an issue.

texas97z

Speedy I was not trying to be arrogant so don't take it that way. But I did indeed in the very first line of my post say "not car in sig, here are specs". In those specs I listed the ford red top injectors. Either way I do appreciate your opinion that the 50 psi should not be a problem. I am replacing the fpr today and hopefully solves all as it is bad.

Injuneer

I've run a stock FPR with an inline booster pump capable of producing 90psi, and the fpr controlled the pressure correctly. What brand is your AFPR?

What brand is your Opti? Even though its new, many replacements are extremely unreliable. For example, did you loc-tite the rotor screws before you installed it?

Since the problem has developed over what appears to be a short period of time, it may be cross-firing. That will cause a backfire into the intake manifold, since it can allow combustion to take place with an intake valve open. By "backfire" do you mean a backfire through the intake manifold, or do you mean a combustion event in the exhaust system?

texas97z

The AFPR is just an Oreilly's reman in which they install a set screw for adjustment. I replaced that yesterday and the fuel pressure is much lower now, around 42psi at idle. That is good to hear that a factory style can do it's job with that much pressure so I am hoping it was just a faulty regulator.

That did not cure backfire or rich condition. Before I changed plugs and wires I was getting both intake and tailpipe back fire. Now it is just tailpipe. I did loc tite the screws for the rotor and it is an AC Delco opti. We changed it out too originally thinking that was the problem regardless of it being brandnew and that made no improvement to situation.

I did notice yesterday that the car smells very strongly of gas on the passanger side only and a slight tick has occured. We have tripple checked the header bolts and they are tight but I am thinking gasket may have burned out and is causing leak? We have reset the valves just in case but it did not improve. I feel pretty strongly that this is the culprit. Also passanger side header gets hot faster and stays hot longer

87bandit

remember having the heavy rich condition could've fouled out your plugs, 45psi at idle is more than my car does at WOT

texas97z

Yeah I checked the plugs yesterday and non on driver were bad and 3 on passanger were fould but they cleaned up. 42 is a bit high your right, and when you take the vacuum line off to simulate WOT it jumped to 48. I was still incouraged though by the fact that it is MUCH lower than the day before with the other regulator. I didn't finish setting the fuel pressure down because the car backfired again so I went back to checking for air leaks and fouled plugs. Thanks for the thoughts, I appreciate it.

I have never seen a header gasket leak bad enough to let air in and fuel out, and the header be tight. But these are the white gaskets from pacesetter on this car, could this be the cause or am I moving in wrong direction?

greenmachinedriver

when you say otsmells strongly of gas on the pasengers side, do you think this might be an exhaust leak? if so it could throw off your o2 causing the plugs to foul. i have never seen anyone use the cheap white paper gakets that come with headders. everyone i know, myself included, has just used factory style gaskets with no issues. if the headder leak is bad enough sometimes you can actually feel the exhaust puff at the point of the leak. if you have a mechanics stethescope you can take off the meatal part and just use the hose and run it around the sealing area, you will hear a leak. you can also just use a pice of vacum hose if you dont have a stethescope.

greenmachinedriver

sorry i forgot i also wanted to mention i have never had good luck cleaning plugs once they are fould out. not shure what kind of plugs you are using but i fould a set of ngk plugs not to long ago by running in failsafe and could not get the car restarted by cleaning the plugs bu one replaced it fired right up.

texas97z

Thanks for the help everyone.
Yeah not my car, just trying to help a guy out. That's why I had to list all the specs. We are going to change out the gaskets on monday. They are NGK plugs and are brandnew so we were trying to avoid buying more and I have had some success in the past with wire brushing the electrode softly and wiping off good with paper towel to get good spark again. But if miss fire continues then we will just have to replace fouled ones. Not expenssive just misfourtune that they only have 150 miles on them. I will take the stethescope on monday and listen but I agree that this probably is the cause. I just wanted to get other opinions incase this does not fix. Been fixing little things on car for over a week and just trying to get it back on the road for him.

texas97z

Still having same problem, here is update-

Replaced fpr with aeromotive after second Oreily brand blew open. No problems now with fuel pressure and keeps backfiring. We changed the exhaust gasket and plugs and it ran good for 20 min's, then started backfiring out the tail pipe again.

So we changed directions and started thinking compression problem. All cylinders hit 120 psi on one stroke except the number 6 cylinder. We pressurized the cylinder and heard no air out the oil tube, exhaust, or intake; but it almost sounded like air coming staright from underneath the head. BTW at that time the max PSI on #6 was 95, way too low in comparrison. So we take the head off and it looks like the metal at bottom of cylinder separated from gasket material (fel pro gasket). We change the gasket after using a square to check for obvious low spots and everything looked good. Put back together and let sit overnight to let all silcone dry. Car ran fine for 5 min and then did it again. A second compression check showed the cylinder would pressurize to 95 psi on first stroke then 120 psi on second, so still not as good as the rest.

We take the head back off, inspect block for gouges or anything. There were none as block just had decks planed when building the stroker. Cylinder walls all look perfect and piston travel is perfect as well. Went ahead and took head to the machine shop and had them plane it to clean it up. Just got it back, but they said they removed only a very very small amount to get it to clean up and should not have had any problems with it. So now I am stumped. again.

It only backfires once car gets warm seems like but it has to be compression related based on what that #6 is reading like. Any help is appreciated guys.

Guest47904

The cylinder in question is not bad. There are several factors that can effect the compression reading. First you must disable the injectors. Second the throttle must be held wide open. But forget all that. You said you have 3 plugs on that side that are fouled.

So either you have an ignition event happening. Or your fuel control is the problem.

So I suggest you get equipment to monitor misfires as OBDII's will tell you which cylinders are misfiring exactly.

Next you should be looking at O2 readings and fuel trims. Since you are having a problem once warm, either you have an electrical breakdown after it warms up. Or you have a problem once the engine goes into closed loop control.

You can force it into open loop by simply unplugging one of the O2s. If the problem is then gone, you know you have a closed loop problem.