99 z28 lag/stutter please help!!

**clewis05** · 05-15-2014, 03:25 PM

I have a 99 Z28 LS1 MS6 Pacesetter LT, 3" OYP, no cats, rear O2 sims, free air box mod. Here lately everytime I start up the car the idle and everything is good, but soon as I take off and lightly press the gas pedal down the car lags/stutters. Then when i come to a stop when its acting like that and push in the clutch the RPMs die way down where it almost dies but doesn't then it'll go back up to normal idle and I smell a strong odor of gas. It doesn't lag like that when I go to WOT and only lasts for a few miles maybe before it works its way out. If I lightly press the pedal and stay like that my SES light will flash. The code it throws is P0131 and the TPS sensor. I replaced both upstream sensors and the TPS sensor and it didn't fix the problem and the SES light is still on. Can anyone give me any advice on what to try next. I really hate taking cars to mechs bc they charge an arm and a leg just to tell me what I already know. Please HELP!!

**pajeff02** · 05-15-2014, 06:05 PM

A flashing SES is bad. Stop driving the car until you diagnose and fix the problem. You shouldn't smell gas -- either something is leaking or you are overfueling the engine by a lot. Did you reset the SES after changing out the TPS? Did you calibrate the new TPS? Check your "ENG SENS" fuse?

**clewis05** · 05-15-2014, 06:42 PM

Whats the flashing mean? I've never heard of something like that. Yes I reset the codes and drove it for a couple miles but then the car would start lagging/sputtering and the SES would flash on and off a couple times and stay on throwing the same codes. No I didn't calibrate the new TPS. I thought it was simple as out with the old and in with the new. How do you go about calibrating the TPS? I looked at the fuse and there is no problem with it.

**theorangeguy** · 05-15-2014, 07:51 PM

Originally Posted by clewis05

Whats the flashing mean? I've never heard of something like that. Yes I reset the codes and drove it for a couple miles but then the car would start lagging/sputtering and the SES would flash on and off a couple times and stay on throwing the same codes. No I didn't calibrate the new TPS. I thought it was simple as out with the old and in with the new. How do you go about calibrating the TPS? I looked at the fuse and there is no problem with it.

Flashing usually means a miss fire somewhere but it could be anything causing it...as Jeff said, don't drive it until you can figure out what is causing it to flash. Miss fires can be minimal or big problems...

**clewis05** · 05-15-2014, 08:09 PM

Thanks for the advice guys. Im going to pull the plugs and have a look at them tomorrow morning. I just replaced the plugs and wires 2 months ago. I used ngk iridium ix plugs and msd wires. the plugs were gapped at .050 I believe. I read that gap is ok for the ls1 but also read to gap at .040. I left them alone at .050 since it said thats what they were pre-gapped for. Could this be the problem? And is there a way to tell if a coil pack is bad? Because if its not the plugs or wires I would assume the coils would be next to look at. Or the injectors??? Appreciate any help on the matter.

**pajeff02** · 05-16-2014, 02:14 AM

I believe the Iridiums like a smaller gap than a standard copper plug. Maybe .040" or so for them rather than the .050". My NGK's are gapped at .054" and I have had zero issues. If it is a misfire, it can be cause by a lean condition, bad plug or wire, or bad coil for the most part. Make certain that you hear/feel two "clicks" when you push each ignition wire onto the coil pack. Also, if you run headers, check to make sure that you haven't burned through a wire.

**0rion** · 05-16-2014, 03:29 AM

another thing to check is the harness going to the #1 O2 sensor on the dirvers side.......might've burned a wire on the header.

**0rion** · 05-16-2014, 03:32 AM

I has a "bad fueling" feel to it whether it's due to the #1 O2 sensor or something else. Check that harness and while you're at it check the vacuum lines for cracks....especially the pcv line where it bends to go into the intake on the passenger side....those like to crack.

**clewis05** · 05-16-2014, 09:07 AM

Ok I'm going to out to the garage and pull my plugs and inspect them, the wires and have a look at my O2 sensor wires too. Hopefully that is what it is but Im skeptical because I triple checked everything when I changed them out a couple months ago. But who knows maybe Im wrong

Ill post an update when Im done.

**clewis05** · 05-18-2014, 04:13 PM

Originally Posted by pajeff02

I believe the Iridiums like a smaller gap than a standard copper plug. Maybe .040" or so for them rather than the .050". My NGK's are gapped at .054" and I have had zero issues. If it is a misfire, it can be cause by a lean condition, bad plug or wire, or bad coil for the most part. Make certain that you hear/feel two "clicks" when you push each ignition wire onto the coil pack. Also, if you run headers, check to make sure that you haven't burned through a wire.

Well I pulled all my plugs and regapped them to .040, checked the wires and checked the O2 wires as well. Put everything back together properly. Plugs still looked really good. Took it out for a test run and it still lagging/sputtering when lightly pressing the gas pedal. Need to know what my next option is?? Is there an easy way to check to see if one or more coils have gone bad without replacing them all?? Any advice at this point would be great. Thanks again guys.

**pajeff02** · 05-18-2014, 05:45 PM

If you start the car with the engine cold and try to drive it immediately, does the engine still stutter? If it does, we can rule out the O2 senders as the PCM runs in open loop until it warms up. This means that it does not accept feedback from the O2 senders to control fueling. After the engine warms up, if the stutter then returns, you may just need to replace the driver side front O2 sender.

If the engine stutters even when cold, then unplug the wire harness to the Mass Air Flow sensor (MAF) and then drive the car and see if the stutter disappears. You will set a MAF code, but that can be cleared without causing any harm. If the stutter disappears with the MAF unplugged, then pull the MAF out of the car and carefully clean it with CRC Spray MAF cleaner. Reinstall the MAF after it dries, plug it back in and see how the engine acts.

[I just went back and re-read your post - I see that you already replaced the O2 senders]

**pajeff02** · 05-18-2014, 05:58 PM

Also, if you want to try and reset the TPS, here is all you have to do:

With the ignition off, unplug the TPS.

Energize the ignition for approximately 10 seconds.

Turn the ignition off and plug the TPS back in.

Start the engine and your TPS should be correct.

**clewis05** · 05-18-2014, 06:22 PM

Ya it does it whether the engine is cold or hot. Ill try driving it with the MAF unplugged but i had pulled that off too and cleaned it when I did the TPS because I thought maybe that had something to do with it, but no luck. Thanks for the heads up with the TPS. Ill def do that. My buddy suppose to be bringing me a better code reader system. Maybe with that it'll tell me more of what is exactly wrong. I had used my other buddies code reader and its the very simple one with a couple buttons on it. Not to much detail to it other than giving me the code. When I drove it earlier I could tell its a misfire somewhere. I've felt it before when I had a bad spark plug on cylinder 8

**pajeff02** · 05-19-2014, 02:21 AM

Another thing to look for is a vacuum leak. The PCV system kind of sucks on our cars and the hoses tend to split right at the connection by the throttle body and at the back corner of the engine. A lean condition can cause a misfire, especially under light throttle conditions when vacuum is at its highest and fuel delivery is low.

**clewis05** · 05-19-2014, 09:53 AM

I had replaced the PCV valve the same time I did the spark plugs and everything. There was nothing wrong with the lines at the throttle body. Where exactly at the back corner do you speak of? Not familiar with vacuum lines like the rest.

***SMWS6TA*** · 05-19-2014, 09:57 AM

Any of the plug wires crack?

Clogged EVAP?

**clewis05** · 05-19-2014, 10:38 AM

No none are. They are all brand new MSD wires. How do you check to see if the EVAP is cloggeed?

**clewis05** · 05-19-2014, 04:17 PM

Originally Posted by pajeff02

Another thing to look for is a vacuum leak. The PCV system kind of sucks on our cars and the hoses tend to split right at the connection by the throttle body and at the back corner of the engine. A lean condition can cause a misfire, especially under light throttle conditions when vacuum is at its highest and fuel delivery is low.

I looked at the vacuum lines and all appear to be fine. No signs of cracks or splits. Any other suggestions?? Just ordered Actron CP9580A scan reader maybe it can tell me a little bit more info on whats going on. Atleast I hope

**pajeff02** · 05-19-2014, 05:25 PM

Any air leak on the engine side of the MAF would be unmetered air entering the engine. Other than the PCV system, you have the MAP sensor at the back of the intake, a vacuum connection in that vicinity, the throttle body, and the intake gaskets. The PCV lines wraps around behind the passenger side rocker cover and crosses over the back of the engine to the driver side rocker cover:

**clewis05** · 05-19-2014, 09:08 PM

Is there any other way other than looking for a crack to see if there is a leak somewhere? Because its a little hard to look at the line behind the intake without actually taking the damn thing off which I assume is a pain in the ass. I looked over the whole piece you have pictured above and it all looks good. Seems to look new still nothing old looking to it. If the MAP sensor was bad I would think it would throw a code for that right? Is there a way to test the vacuum pressure assuming there is a specific PSI that it should be at when at idle? I do appreciate the help you're providing me man. I'm just trying to do everything myself without having to take it to a mechanic