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Discussion Starter #1
Let me start by prefacing that my car is a 1999 3000gt SOHC Automatic.

I am currently experiencing several problems with my car after doing a rebuild on the engine, including:
  • Car is running pig rich, leading to a strong gas smell as well as bogging down
  • Idle is rough and surges from 600 to 1500 RPM
  • When not under load the car stutters through RPMs when slowly accelerating, but has a smooth rev when going WOT
  • Car begins to severely stutter at around 2000 RPM
  • CEL is on and has codes P0340, P0335, and P0161
The car was not running when I bought it which led to me rebuilding the engine.
I have replaced the MAF sensor, MAP sensor, TPS, and Crankshaft position sensor.
I also have brand new spark plugs, plug wires, distributor cap and rotor, PTU, and Coil (and yes my car is the SOHC so it does have a distributor).
I have tried testing many things and swapping out sensors, but I don't want to just throw money at the car.
I want to get the car running right before I go back to school, and I have tried searching through the forums already for things to do. Any help on what could be the issue or suggestions of what to try is greatly appreciated.
 

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93 NA ATX 3000gt DOHC
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Double check plug wires to cylinders, as it’s easy to make error with the weird cap layout. See diagram below.

Edit: An after thought, if you have original cap check to see if new cap has wire positions marked in same order. There are so many Mitsubishi distributor caps that might fit with different wire locations, it’s possible you have wrong cap.
SOHC dist cap wires & fire order.jpeg Click on image to enlarge
 

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Discussion Starter #3
Double check plug wires to cylinders, as it’s easy to make error with the weird cap layout.

Edit: An after thought, if you have original cap check to see if new cap has wire positions marked in same order. There are so many Mitsubishi distributor caps that might fit with different wire locations, it’s possible you have wrong cap.
View attachment 289241 Click on image to enlarge
Thanks for the suggestion. All of the plug wires go to the correct locations; I double checked with the old cap and it was the same.
I did pull the front 3 spark plugs and noticed that cylinder 3 had a considerable amount of carbon on it, while cylinders 1 and 5 had none.
 

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I did pull the front 3 spark plugs and noticed that cylinder 3 had a considerable amount of carbon on it, while cylinders 1 and 5 had none.
Did cylinders 1 and 5 seem wet with fuel? No carbon could indicate not firing or no fuel being delivered.
 

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They didn't seem wet with fuel. They had minor wear on them and were white on the electrode.
That could indicate fuel deliver problem which could be cam position sensor, which best I can tell is located in distributor and not sure it can be replaced without replacing whole distributor (no personal experience). It could also be injector problem on those two cylinders, but lack of fuel isn’t likely since #3 cylinder between the two is getting fuel.

Did you have chance to look at diagram I added about same time you last posted. It indicates improper wire routing can cause similar problems as you describe. I can also tell you from experience that leaking plug wires can cause similar problems and won’t always be noticed at WOT. You might pull one plug wire from cap at a time when at idle and see if any don’t make a difference.
 

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Discussion Starter #7
That could indicate fuel deliver problem which could be cam position sensor, which best I can tell is located in distributor and not sure it can be replaced without replacing whole distributor (no personal experience). It could also be injector problem on those two cylinders, but lack of fuel isn’t likely since #3 cylinder between the two is getting fuel.

Did you have chance to look at diagram I added about same time you last posted. It indicates improper wire routing can cause similar problems as you describe. I can also tell you from experience that leaking plug wires can cause similar problems and won’t always be noticed at WOT. You might pull one plug wire from cap at a time when at idle and see if any don’t make a difference.
I had a look at the diagram just to make sure the way I had routed everything was correct and I saw the note about incorrect routing.

The only way to replace the camshaft position sensor is to replace the whole distributor unfortunately.

I'll go ahead and try what you said about pulling them one at a time tomorrow and get back to you. I'll also pull the cylinder 1 spark plug after its ran for a bit to see if its wet with fuel.

Another thing to note is that when the car is in limp mode (MAF unplugged) the idle doesn't surge, and there is no stuttering as I accelerate.
 

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Another thing to note is that when the car is in limp mode (MAF unplugged) the idle doesn't surge, and there is no stuttering as I accelerate.
There might be a clue there will have to think on that one awhile and will look to see if you find anything while pulling plug wires tomorrow.
 

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There might be a clue there will have to think on that one awhile and will look to see if you find anything while pulling plug wires tomorrow.
Maybe I should have ask how long has engine has been run since new plugs were installed. Now thinking maybe cylinders 1 & 5 are working correctly and white electrode porcelain is just from little use while #3 is flooding cylinder causing carbon build up. The idling test (pulling wires) might shed some light on this.

Since you indicated running pig rich (gas smell), it makes sense that it runs better with MAF unplugged as ECU would lean mixture thinking engine was basically at idle all the time. If your idle/wire test indicates a problem with #3, we could then assume it or/and more cylinders are being over fueled for some reason. Then it would just require figuring out how to narrow that down.
 

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Discussion Starter #10
Maybe I should have ask how long has engine has been run since new plugs were installed. Now thinking maybe cylinders 1 & 5 are working correctly and white electrode porcelain is just from little use while #3 is flooding cylinder causing carbon build up. The idling test (pulling wires) might shed some light on this.

Since you indicated running pig rich (gas smell), it makes sense that it runs better with MAF unplugged as ECU would lean mixture thinking engine was basically at idle all the time. If your idle/wire test indicates a problem with #3, we could then assume it or/and more cylinders are being over fueled for some reason. Then it would just require figuring out how to narrow that down.
The spark plugs haven't been ran for long. Maybe 100 miles? And then however long I've been idling the car trying to figure out this issue.

I did the idle/pulling test that you described and found that cylinder #3 had the most erratic change in the idle and revs. The rest of the cylinders just made the car have a fast stuttering on top of the already bad idle.

I pulled the spark plugs after doing the testing to find that none of the plugs were wet with fuel, meaning that there is still spark in each
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cylinder but just too much fuel.

I attached some pictures of the plugs as well.
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WOW… First I got to say that’s the worst looking set of plugs I’ve ever seen for 100 or so miles. Much different than I expected, was thinking 1 & 5 might look near new and 3 might look like 1 & 5 does now. They all have been getting to much fuel and 3 looks to be getting some oil also, by the shine on electrode porcelain as excess fuel will leave a flat sooty look.

When you say “cylinder #3 had the most erratic change in the idle and revs” does that mean idle got rougher and rpm’s went down (like maybe near stalling) or did idle roughness improve and rpm’s go up? The fast stuttering on other cylinders sounds like evidence of stead misfire which is what you would expect when wire pulled.

Where to start… I would start with compression test of at least those front three cylinders to compare 1 & 5 to 3. Then I would reinstall plugs and use timing light to see if spark advance at harmonic balancer and indicator on T-belt cover shows something ~ 15 degrees advance when idling.

Also are you sure the mechanical timing of cams to crank of belt is correct?
 

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Discussion Starter #12
WOW… First I got to say that’s the worst looking set of plugs I’ve ever seen for 100 or so miles. Much different than I expected, was thinking 1 & 5 might look near new and 3 might look like 1 & 5 does now. They all have been getting to much fuel and 3 looks to be getting some oil also, by the shine on electrode porcelain as excess fuel will leave a flat sooty look.

When you say “cylinder #3 had the most erratic change in the idle and revs” does that mean idle got rougher and rpm’s went down (like maybe near stalling) or did idle roughness improve and rpm’s go up? The fast stuttering on other cylinders sounds like evidence of stead misfire which is what you would expect when wire pulled.

Where to start… I would start with compression test of at least those front three cylinders to compare 1 & 5 to 3. Then I would reinstall plugs and use timing light to see if spark advance at harmonic balancer and indicator on T-belt cover shows something ~ 15 degrees advance when idling.

Also are you sure the mechanical timing of cams to crank of belt is correct?
Definitely agree with you on the condition of the spark plugs. What's weird is when I took them off the other day they didn't look as bad.

When unplugging cylinder #3 the idle got rougher and rpms went up.

I'm off of work tomorrow so I'm gonna take off the timing covers to double check that all of the timing marks line up, and I'm gonna have my friend come over with his datalogger, compression tester, and timing light to see if he can help diagnose the problem.

Thanks for all of the help thus far.
 

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I'm off of work tomorrow so I'm gonna take off the timing covers to double check that all of the timing marks line up,
Make sure crank is on mark also or if you don't want to pull harmonic balancer you can just check to see if notch in balancer is lined up on TDC indicator on belt cover, should be indicated by "0" or a "T" not sure which cause some are one and some the other.
 

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Discussion Starter #14
Make sure crank is on mark also or if you don't want to pull harmonic balancer you can just check to see if notch in balancer is lined up on TDC indicator on belt cover, should be indicated by "0" or a "T" not sure which cause some are one and some the other.
Cool. I'll take a look on the harmonic balancer and then most likely check underneath as well.
 

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Cool. I'll take a look on the harmonic balancer and then most likely check underneath as well.
If you have to get balancer off to verify cog gear tooth to oil pump mark, also check reluctor plate notch is line up with tooth marked on top with counter sunk dot. see attached photo of one from DOHC.
Crank cog gear.jpg

Added note this shows crank cog gear/reluctor plate assembly is one full tooth to far advanced for correct crank/cam timing, but reluctor plate is in correct alignment to cog gear.
 

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If you have to get balancer off to verify cog gear tooth to oil pump mark, also check reluctor plate notch is line up with tooth marked on top with counter sunk dot. see attached photo of one from DOHC.
View attachment 289246
Added note this shows crank cog gear/reluctor plate assembly is one full tooth to far advanced for correct crank/cam timing, but reluctor plate is in correct alignment to cog gear.
Just got the timing covers off. All of the marks line up exactly. I've attached pictures of them.
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If you have to get balancer off to verify cog gear tooth to oil pump mark, also check reluctor plate notch is line up with tooth marked on top with counter sunk dot. see attached photo of one from DOHC.
View attachment 289246
Added note this shows crank cog gear/reluctor plate assembly is one full tooth to far advanced for correct crank/cam timing, but reluctor plate is in correct alignment to cog gear.
Just got the timing covers off. All of the marks line up exactly.
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I don’t know what to say for sure about crank timing in those pictures. Never done late model SOHC 97-99 belt and all others I’ve seen has had timing mark on pump to the right side, so all I can say if that’s only indicator on pump it appears to be right. Can’t see if reluctor plate has a notch that matches that tooth of gear, but if it does that would also confirm that is correct as right side of reluctor plate appears to be in correct position at TDC as on the DOHC engines are.

Did you get compression checks done?
 

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Discussion Starter #18
I don’t know what to say for sure about crank timing in those pictures. Never done late model SOHC 97-99 belt and all others I’ve seen has had timing mark on pump to the right side, so all I can say if that’s only indicator on pump it appears to be right. Can’t see if reluctor plate has a notch that matches that tooth of gear, but if it does that would also confirm that is correct as right side of reluctor plate appears to be in correct position at TDC as on the DOHC engines are.

Did you get compression checks done?
Yes the reluctor plate was matched with the dot on the crank gear. The manual indicates that those are the marks to line the cams up with.

I didn't happen to do compression tests on the cylinders yet.
 

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Yes the reluctor plate was matched with the dot on the crank gear. The manual indicates that those are the marks to line the cams up with.
OK….. That sounds good, just looking for some major problem since it seems as though an electrical type problem wouldn’t cause some of strange problems you seem to have. Like when pulling #3 wire and rpm’s going UP, getting rough and rpm’s going DOWN would be expected and the fact that when other wires were pulled they didn’t impact engine roughness as much as I expected they would. That along with plug visual differences just seems strange.

Another thing I noticed is no roll pin in cam gear to index harmonic balancer, I guess yours index’s on the key, if so the notch on back edge of balancer should also line up to TDC indicator on belt cover when engine is in that position.

As far as electrical possibilities to cause that rich state. Since the TPS was replaced, was position of that set per manual instructions?

All most acts like ECU isn’t transitioning from open loop to close loop and is continuing to over fuel.
 

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Discussion Starter #20
This issue is so bizarre. It feels like either the ignition timing is off, leading to rich conditions, or the injector timing is off so there is too much fuel in the cylinders.

I wish I had someone in my area that I could swap ECUs with to see if that's the problem. Its also impossible to find a distributor for my car if the Camshaft Position Sensor is bad.
 
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