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Discussion Starter #1
I've been having a blast driving my TT since I got it back together after a head gasket repair / 60k service until today. I did a quick half throttle pull to show off the new BOV to my girlfriend and ever since the car has had what feels like a constant misfire. It genuinely feels like the car is running on 5 cylinders. I got to about 10psi of boost when this happened. My first suspicion was that I blew off a vacuum line, but I can't find where any are loose. She was running absolutely perfectly before this happened and I just got done with hours upon hours of labor putting the car back together properly so I'm a bit bummed out.... Can anyone point me in the right direction to start looking for the cause tomorrow? A boost leak test is the first thing I plan to do....

Edit: I have fully equipped my car with everything I should need to data log the car with Blackstealth's converter and the Torque app, but I don't know what I'm looking at well enough to sniff out a problem using it. No CEL are showing either.
 

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Did a boost leak check today. Had a tiny leak from one of the vacuum hoses on top of the throttle body. I fixed that but the problem still persists. The car runs and idles fine, but it feels like it's doing it with one less cylinder. The whole car shakes when you accelerate. New spark plugs, wires, and coils. Spark plugs gapped at .38. This one has me pretty stumped. I can try to capture some logs if one of you could be so kind as to help me know what I'm looking at.
 

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Check out my thread here for getting a good setup for datalogging and some key parameters to look at:

https://www.3si.org/forum/f35/datalogging-using-torque-pro-app-blackstealth-s-obd1-converter-discussion-q-811626/

Are you logging using bluetooth OBD2 converter? If so, you wont get much speed/resolution.

In terms of your misfire, is the RPM gauge off? If its PTU related, typically your RPM gauge will read wrong, but i believe that would also affect 2 cylinders (1 coil) at once. Any pops or other odd noises when the issue first started?

If you think it is down a cylinder, pull off the plug wires one at a time at idle and see if one of them does not make a difference. Slim possibility, but its possible one of the plug wires came off on the plug itself? If you can isolate it to a specific cyliner, check that wire, and the plug if its in the front. If its in the rear you may need to result to pulling the plenum again, and while you are in there you should probably just do a compression check.

Basically, isolate it to a specific cylinder if you can, then make sure that cylinder is getting fuel and spark. You can also check injector harnesses. If you just put everything back together its worth going over all electrical connections as well.

If you have a wideband, do your AFRs look way off now that the misfire is there?
 

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Discussion Starter #4
Check out my thread here for getting a good setup for datalogging and some key parameters to look at:

https://www.3si.org/forum/f35/datalogging-using-torque-pro-app-blackstealth-s-obd1-converter-discussion-q-811626/

Are you logging using bluetooth OBD2 converter? If so, you wont get much speed/resolution.

In terms of your misfire, is the RPM gauge off? If its PTU related, typically your RPM gauge will read wrong, but i believe that would also affect 2 cylinders (1 coil) at once. Any pops or other odd noises when the issue first started?

If you think it is down a cylinder, pull off the plug wires one at a time at idle and see if one of them does not make a difference. Slim possibility, but its possible one of the plug wires came off on the plug itself? If you can isolate it to a specific cyliner, check that wire, and the plug if its in the front. If its in the rear you may need to result to pulling the plenum again, and while you are in there you should probably just do a compression check.

Basically, isolate it to a specific cylinder if you can, then make sure that cylinder is getting fuel and spark. You can also check injector harnesses. If you just put everything back together its worth going over all electrical connections as well.

If you have a wideband, do your AFRs look way off now that the misfire is there?
I am using a USB OBD2 converter run directly in to the back of my android head unit. The responsiveness on this adapter is nearly instant and updates very quickly.

My RPM gauge is working properly so I'll assume that isn't the problem for now. It was replaced by the previous owner in 2015. As far as any other noises when the car started acting up, there were none. I let off the throttle after a half throttle pull and upon the next time I gave it gas the car started acting like it was missing.

I individually disconnected the plug wires from the coils one at a time and the idle bogged down equally for every one of them, so I guess all cylinders are in fact firing.

I do not have a wideband installed yet. It was a consideration for the future though.

I'll go ahead and check out your datalogging thread and see what I can come up with. Thank you!
 

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Ah if you are using wired USB your speed will be great. You should be able to look at numerous PIDs in torque and get real time info. Its the bluetooth that is super laggy.

Seems a little odd that it's even across all cylinders... I was suspecting one plug or one injector type problem. Getting torque setup to monitor and log some key PIDs might provide some clues. Then drive around and watch things in real time and do some logs.

Any other symptoms you can describe? Does the car smell like it's running more rich now? Same behavior on cold start and hot start? I wouldn't try driving it too hard but if you roll into boost does it clear up or behave the same?

The more I think about it one thing that can affect all cylinders evenly is timing. I would definitely check timing belt, make sure it's not loose or didnt skip a tooth, and perhaps check base timing as well. This would definitely make it seem like it was misfiring.
 

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Discussion Starter #6
Ah if you are using wired USB your speed will be great. You should be able to look at numerous PIDs in torque and get real time info. Its the bluetooth that is super laggy.

Seems a little odd that it's even across all cylinders... I was suspecting one plug or one injector type problem. Getting torque setup to monitor and log some key PIDs might provide some clues. Then drive around and watch things in real time and do some logs.

Any other symptoms you can describe? Does the car smell like it's running more rich now? Same behavior on cold start and hot start? I wouldn't try driving it too hard but if you roll into boost does it clear up or behave the same?

The more I think about it one thing that can affect all cylinders evenly is timing. I would definitely check timing belt, make sure it's not loose or didnt skip a tooth, and perhaps check base timing as well. This would definitely make it seem like it was misfiring.
I just set the timing less than 100 miles ago during a 60k service. It was still dead on when I checked today. However, I have not set my ignition timing after having to remove the CAS to have my heads resurfaced. I tried to put it back exactly where it were before I removed it. Do you think incorrect ignition timing could make the car run like garbage constantly? It was completely fine for the 100 miles or so I put on it after reassembly. If so I'll need to buy a timing light.
 

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What happens if you go back to the old BOV? if that was the only modification and it started doing that..
 

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However, I have not set my ignition timing after having to remove the CAS to have my heads resurfaced. I tried to put it back exactly where it were before I removed it. Do you think incorrect ignition timing could make the car run like garbage constantly?
Short answer is yes, absolutely timing being off will make the car run terrible.

Now as to why it ran fine until you did a pull, I am not sure unless the CAS was not fully tight or something and moved when you did that pull? Did you make marks during your previous reassembly so you can tell by inspection if it moved since then? Also check the electrical connection on top of it; i had mine come loose and it car would all the sudden hit a wall like it was fuel cutting because that plug had backed itself off. Just pushed it back down and that went away. Not sure a loose plug would cause your issue though unless it was somehow causing timing to be misread.

In any case, it is probably safest so set your base timing, and it would be another thing to check off the list.
 

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Discussion Starter #10
Short answer is yes, absolutely timing being off will make the car run terrible.

Now as to why it ran fine until you did a pull, I am not sure unless the CAS was not fully tight or something and moved when you did that pull? Did you make marks during your previous reassembly so you can tell by inspection if it moved since then? Also check the electrical connection on top of it; i had mine come loose and it car would all the sudden hit a wall like it was fuel cutting because that plug had backed itself off. Just pushed it back down and that went away. Not sure a loose plug would cause your issue though unless it was somehow causing timing to be misread.

In any case, it is probably safest so set your base timing, and it would be another thing to check off the list.
The connector is secured on there really well. I don't think that is the issue.
Here is a little video I took of the way it sounds. You can barely tell through the video, but it runs perfectly, just with a weird miss.


See what you think of that. Hopefully I can get a timing light soon to get that out of the question.
 

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The connector is secured on there really well. I don't think that is the issue.
Here is a little video I took of the way it sounds. You can barely tell through the video, but it runs perfectly, just with a weird miss.

https://www.youtube.com/watch?v=-Q687e4MlRg

See what you think of that. Hopefully I can get a timing light soon to get that out of the question.
Just reread the tread and watched the video a few times... no great ideas unfortunately. I think timing is definitely the next thing to check; im struggling to think of other things that would suddenly affect all cylinders evenly.

It sounds like there are some small pops in the exhaust after letting off from a rev. Were those there before?

Check your O2 voltages in torque when you rev and whatnot to see if those are responding normally/evenly (going to 1 for rich when revving and to 0 on decel). They should also cycle around 0.5 to achieve 14.7 AFR when you are in closed loop (warm idle or warm low throttle cruising).

You could also try unplugging your MAF to see how the car responds.
 

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The connector is secured on there really well. I don't think that is the issue.
Here is a little video I took of the way it sounds. You can barely tell through the video, but it runs perfectly, just with a weird miss.

https://www.youtube.com/watch?v=-Q687e4MlRg

See what you think of that. Hopefully I can get a timing light soon to get that out of the question.
So i just typed a post and i think it disappeared... anyways i watched the video a few times and reread this tread. I dont have any great ideas at the moment but will spitball a few more things. Can't think of much that would affect all cylinders evenly in that way...

Timing check would still be top priority IMO.

Also, watch O2 sensor voltages on torque and see if they respond as expected (to 1 for rich on a rev and to 0 on decel when no fuel is injected). They should also cycle around 0.5 to matain 14.7 AFR when in closed loop (warm idle or warm light throttle cruising).

You could also try unplugging the MAF and see how the car responds. I believe it just defaults to fuel tables when you do that since it cant read airflow.

How is the fuel filter? Very old?
 

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edit: deleted duplicate, sorry for that, browser issues.
 

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weird your car behaved as mine did after i did some logging under throttle this week. Backstory is that my car was running great, short drive to a straight stretch of road, did my run and a log, short drive back home under normal driving, car continued to run fine. Parked the car, came back the next day to run an errand, started it up and it had a miss just like your video. I assumed that i had a boost leak or perhaps left unburnt fuel in a cylinder from the night before, but I was in a hurry, so just sat a few minutes revved it up and eventually the idle smoothed out. I've not had a need to start it again and see if it comes back.

I do plan to pull my ECU and replace the caps, and I wondered if "spirited driving" puts extra strain on the ECU caps...
 

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Discussion Starter #16
weird your car behaved as mine did after i did some logging under throttle this week. Backstory is that my car was running great, short drive to a straight stretch of road, did my run and a log, short drive back home under normal driving, car continued to run fine. Parked the car, came back the next day to run an errand, started it up and it had a miss just like your video. I assumed that i had a boost leak or perhaps left unburnt fuel in a cylinder from the night before, but I was in a hurry, so just sat a few minutes revved it up and eventually the idle smoothed out. I've not had a need to start it again and see if it comes back.

I do plan to pull my ECU and replace the caps, and I wondered if "spirited driving" puts extra strain on the ECU caps...
Please let me know if this fixes it! I have an ECU with fresh caps in the car that I put in right before I tore it down to do the head gasket. Hopefully it's not another screwed up ECU because that's one of the issues the car had when I bought it.
 

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So i just typed a post and i think it disappeared... anyways i watched the video a few times and reread this tread. I dont have any great ideas at the moment but will spitball a few more things. Can't think of much that would affect all cylinders evenly in that way...

Timing check would still be top priority IMO.

Also, watch O2 sensor voltages on torque and see if they respond as expected (to 1 for rich on a rev and to 0 on decel when no fuel is injected). They should also cycle around 0.5 to matain 14.7 AFR when in closed loop (warm idle or warm light throttle cruising).

You could also try unplugging the MAF and see how the car responds. I believe it just defaults to fuel tables when you do that since it cant read airflow.

How is the fuel filter? Very old?
Firstly, thank you for being so helpful! I really appreciate you trying to help me get this thing running correctly again.This is my first Stealth and I've only had it 6 months.

Secondly, I'll check out the O2 sensor readings when I get some free time. i'll also try unplugging the MAF. I also have a spare working MAF I could swap in and try. The fuel filter was replaced in 2015 less than 1000 miles ago according to the service history. As for the pops, it did not pop at all before I got rid of all the cats. the few miles it was running perfectly after being put back together without cats it popped a little, but not as much. I assumed it was due to the lack of cats.
I plan to pull the plugs and check them for oil/damage since that was also suggested to me. Hopefully it's nothing catastrophic. I'm not about to drive this car any further than a mile until I get it fixed so I don't make things worse if it is caused by detonation.
 

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Please let me know if this fixes it! I have an ECU with fresh caps in the car that I put in right before I tore it down to do the head gasket.
I don't plan on swapping ECU caps until the winter when my car is off the road. I actually have the caps and a spare ECU to test. I'm like you in that I'm not interested in long trips until I get this sorted out.

Frankly i believe that the bulk of the driveabiity issues being posted are ECU related. Sadly, guys will be giving up and selling/parting cars when it starts to happen.
 

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I don't plan on swapping ECU caps until the winter when my car is off the road. I actually have the caps and a spare ECU to test. I'm like you in that I'm not interested in long trips until I get this sorted out.

Frankly i believe that the bulk of the driveabiity issues being posted are ECU related. Sadly, guys will be giving up and selling/parting cars when it starts to happen.
That's exactly how I got my car for so cheap. The ECU was almost toast. Random misfires, wouldn't start half the time, bucking under boost. It was awful. I took a chance and bought it. Sure enough, all of that was because of the ECU.
 

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Firstly, thank you for being so helpful! I really appreciate you trying to help me get this thing running correctly again.This is my first Stealth and I've only had it 6 months.

Secondly, I'll check out the O2 sensor readings when I get some free time. i'll also try unplugging the MAF. I also have a spare working MAF I could swap in and try. The fuel filter was replaced in 2015 less than 1000 miles ago according to the service history. As for the pops, it did not pop at all before I got rid of all the cats. the few miles it was running perfectly after being put back together without cats it popped a little, but not as much. I assumed it was due to the lack of cats.
I plan to pull the plugs and check them for oil/damage since that was also suggested to me. Hopefully it's nothing catastrophic. I'm not about to drive this car any further than a mile until I get it fixed so I don't make things worse if it is caused by detonation.
Happy to help. My car is running great now after some recent maintenance (hoping it stays that way for a while) so i have to spend less time trying to post/read about solving my own issues, lol.

Someone else was just posting about MAF issues as well, where unplugging the MAF made the car run better. That could be a very simple test (as would swapping a different one in).

Yes, the pops are probably from no cats, not necessarily due to the misfire issue.

Even if its not at the top of the list, compression test may be in your future after checking MAF and timing.

PHP:
That's exactly how I got my car for so cheap. The ECU was almost toast. Random misfires, wouldn't start half the time, bucking under boost. It was awful. I took a chance and bought it. Sure enough, all of that was because of the ECU.
I think this is how most ECUs go... but mine was totally different. The car was 100% fine. I leave my house one day, get about 2 miles away and am sitting at a stoplight when all the sudden i see smoke from under the dash and the car dies. Cranks but doesnt start. Smell burning electronics. Get the car towed home, do some research, pull the ECU and it is totally fried, black board and all. It could not even be repaired so i ended up paying around $500 for a reman one (this was about 8 years ago). Knock on wood, but that one has been fine ever since. My ECS (suspension) computer also burned out in a similar fashion.
 
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