Hello all!

I’ve ran into a problem with my 95 LT1 M6 convertible after installing some Hooker long tubes. When I’m cruising around 2000 rpm or less, it does fine. However, if I punch the gas it would stumble and start to almost plateau on power with pops from the exhaust. It also starts humming some resonance from the exhaust and it doesn’t want to go. If I downshift, it’ll pick up but it doesn’t feel like there’s as much power there as before.

At higher rpm it seems to be okay, but sometimes I can feel by the SOTP dyno that something is off.

When I installed, I did new NGK G-Power plugs, new ACDelco wires, UMI EGR block off at the intake. I unplugged the EGR solenoid until a tune and plugged the vacuum line. Also unplugged the AIR pump. This threw a CEL as expected. I then plugged the EGR back in, CEL went away but same symptoms.

I then also tried to unplug the O2 sensor in hopes of triggering a base tune in “limp mode,” but I didn’t get a CEL here and still ran bad.

Im hoping someone may have some tips
on what’s next here. I’m wondering if it’ll need a tune for the EGR delete and if the PCM is still commanding the timing adjustments even though the EGR was unplugged and a CEL was thrown, or if there’s something else I’m missing entirely and it’s a new problem I created during install.

Any tips are appreciated. Thanks!

 

Is the O2 sensor in the header close to head or further down in collector pipe?

 

There are three of them on the header setup. Reversed from stock, the drivers side has 2 O2 sensors; one on cylinder 7, then one in the collector. The passenger side just has one in the collector. The sensor I unplugged trying to trip the CEL was the passenger side but it didn’t do anything.

 

Sounds like it's lean under load.

 

This was my assessment too, but I’m curious the cause. My thoughts were either the O2 sensors have no clue what’s going on, the PCM just isn’t tuned right for these headers and it can’t compensate enough, or the EGR circuit is still adjusting trims to account for exhaust gas that would be going into the cylinders but isn’t now.

 

Did some diag tonight when I got home. Found out I didn’t tighten cylinder 1’s plug, and after trying to tighten it down with the headers on, I see why I didn’t and forgot about it. I was able to take it out by hand when I discovered that.

It still has its hiccup around 2000 rpm though, but not as bad. If I’m cruising around 1/4 throttle and I push the gas in to 1/2-3/4 throttle, it won’t accelerate and it drones from the exhaust. If I put it to the floor, it stumbles out of it and rips like it should. This was with the EGR solenoid unplugged and the CEL on, hoping to throw it into limp mode.

Could the PCM be using base fuel trims and it’s just too lean for the headers? Do I need a tune? I’ve never tuned before but if anyone has pointers I’m willing to learn. Local tunes here in Indy are about $400-$600 if they even tune LT1s.

 

Needs a tune… and a wideband. Short of proper tuning, you may try adjusting fuel pressure but the transient enrichment needs ECU flash or aftermarket management system.

 

How would I go about adding a wideband o2 sensor? Is it worth it to talk to Solomon at LT1 PCM Tuning and let him know my setup for a tune? I think he was at $200 for an email tune.

 

Re-reading this made me realize something. I have an aftermarket FPR on there that I didn’t install. I’ve never verified fuel pressure on this car and it’s always had a bog around 3000 rpm until WOT. Wondering if someone’s messed something up in the past. Going to purchase a fuel pressure gauge and verify fuel pressures at ON, idle, and revving. Thanks for the tips here on this.

 

I have a 95 also and have learned to tune my car. If you need help with tuning let me know because I live a couple hours north of you and can give you hand with tuning it. It's not difficult, just tedious.

 

Might take you up on that. I planned on buying either the Tunercat Pro bundle or just buying the cable and getting a free software from Solomon’s suggestions.

I’m also planning on verifying fuel pressure at the rail this evening with my newly acquired fuel pressure gauge!

 

Did the fuel pressure test, no good. Primed the pump and it reaches about 30 psi and drops to 0 immediately after the pump is done priming. FPR isn’t wet so I don’t believe it’s that. Going to check injectors and fuel pump this week.

 

Are you going to do a pinch off test?

 

Yeah I just found that thread on how to do it. The plan is to do that tomorrow likely.

 

I had an Aeromotive FPR that doesn’t hold pressure. I didn’t install, car came with it. I didn’t feel great about it based of all the bad reviews so I replaced with an AC Delco oem FPR. New pump on it too.

Took it for a drive and same issue, around 2000-3000 rpm it will randomly stumble and lose power, drone from the exhaust as well. This is all at about 1/2-3/4 throttle. If I floor it, seems to stumble out of it but not pull that hard. Next on the check list is a tune. Stay tuned for the update (pun intended).

 

Did the fuel pressure test change in any way?

 

Yes it did, it now jumped to 40 while priming and then sat at 35 after the pump shut off, it didn’t bleed down. It starts a lot easier now. With the Aeromotive FPR it would always take a couple cranks to start it. Now it’s almost instant.

I found some other forums on the F-Body sites that mentioned that the Aeromotive actually doesn’t hold pressure, and that the original version of it had issues with “new formulations of gas.” I’m not sure how old the one is that was on there so I decided just to replace it anyway.

 

Somehow I doubt that simply the addition of some headers is causing this. Seems like something else is going on.

What's happening is that at WOT the PCM ignores the O2 sensors as it goes into Power Enrichment... which is why after the initial stumble clears, the car takes off... which also indicated that this wasn't a fuel pressure issue, otherwise you'd be going really lean at WOT.

Figure this... if the headers were such an upset to the factory calibration that the PCM ran out of adjustment range on the fueling, there's no way the WOT fueling wouldn't also be way off. Since the PCM only uses a pre-programmed AFR during WOT, you'd feel it there too.

I'm not an expert on 94-95, but nothing that I can see in the code has PCM adjusting fueling based on EGR. The EGR does slow down the rate of burn in the combustion chambers, so if that's not there, you would increase your propensity to get into knocking/pinging. In which case the PCM could be pulling some timing out if the knock sensors are detecting that. But again, you'd see this at WOT more than you would at part throttle, so I'm inclined to discount this as well.

Have you pulled any spark plugs to see what they're showing? Often times they have a pretty good story to tell.

 

https://ecmhack.com/tutorials/tuning-for-headers/

 

Plugs looked good when I last pulled them. Didn’t see any signs of detonation, fuel, fouling. All seemed normal there. But I’ve driven it quite a few more times since I initially checked them, so possibly later today or tomorrow I’ll pull the plugs again and snap some pictures to post here.

I agree it likely wasn’t a fuel pressure issue from the beginning, but it was definitely something I needed to take care of. That long crank was annoying and worrisome at times, haha! A guy can only hope something simple like that is the answer.

Let me ask this though, it’s only at 1/2-3/4 throttle and at 2500ish RPM, which seems to be EGR operating range. I’ve heard that the EGR will still try to compensate timing based on MAP/RPM/temp. But at the same time if the EGR solenoid is unplugged and can’t control the valve, I would think this would null that data and still adjust accordingly based on my knock sensors as you stated. I guess what I’m asking is could the PCM still be adjusting timing thinking that there’s exhaust gas entering the combustion chamber?

All in all though, thanks for your info there! Super helpful for me, who doesn’t know a whole lot about fuel injection systems and tuning.