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Hello all, I'm new to this site so be easy ……...

I have a 91 non turbo with 30,000 original miles on it. It has developed a misfire recently. I sent the ECM in a couple of months ago because of leaky transistors, it was misfiring before and still is. I tested the coils and they test fine, replaced plugs wires, plugs and TPS. Readjusted the SAS and TPS and checked the Power TR Unit and checked the timing. Nothing has worked so far. A weird thing though, I put my timing light on each plug wire and every one has a very short period of hesitation, no light. it is erratic, no pattern to it at all. Never get a check engine light?

Thanks!
 

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Welcome!

How did you determine that it is a misfire? Personally, I would invest into a datalogger to gain better insight. Typically, I see random misfires caused by vacuum leaks. If it is more isolated to a cylinder(s), then it could be an injector(s). Miles aside, old is old, so there are quite a few possibilities.
 

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Thanks for your response, where would be a good place to get a Data Logger?
You can hear and feel the misfire when its idling as well as when your cruising. I live in California and it wont pass smog running like it is. It has to maintain 15 +/- 1 MPH in second gear for 30 seconds and the misfire is so bad it won't allow this to happen because you have to keep accelerating then decelerating too much. It will quit then come back and so on and so on until the point to were it fails. One good thing is it burns clean as can be!
 

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There are a few options for a logger or scanner depending on preference and how much you want to invest into one. Where in CA do you reside? It might be easier to have someone local plug-in and see what is up.

I still think it might be vacuum leaks though. Is the MAFS dirty? When was the fuel filter last changed?

-sent from my Galaxy Note 9
 

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I'm in Riverside. After reading your post this morning I sprayed carb cleaner on the EGR valve and around the intake manifold, When I spray the EGR the idle does raise very little, maybe 50 rpms I'm guessing, its not much. I've checked all the vacuum lines I can find and they appear ok. I have never changed the fuel filter but the car doesn't lack power or hesitate like its a fuel issue. When you gas it, it go's, the car actually stills halls ass, but then when you get to cruising, it's starts the missing and depending on your speed it gets a little jerky.
 

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Ah, well that certainly suggests some things. Did you reuse all gaskets when you changed the plugs and wires, or did you install new ones? If the former, did you apply a dressing e.g. Permatex copper

You can also test the solenoid for the EGR valve if you have access to a handheld vacuum pump. If it was stuck open, you would have other issues even just idling, but it does seem to be affecting the conditions where it would be in operation - coincidence or not.

-sent from my Galaxy Note 9
 

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I forgot, the MAFS looks brand new still, the screws to get the screen off were kind of stuck or something, I couldn't get them out but it looked good otherwise. I believe it's working because I started the car without it hooked up accidently and it ran like crap until I plugged the wire's back in. While it was unplugged I did notice the RPM's dropped to about 550 and the fluctuation was still there but it was more like the motor was surging and not a blubbery miss.

To be honest I don't remember what I did, I don't remember buying a gasket but I don't remember putting any sealant on it as well...….. I'll just redo that and see if that helps anything.

I can watch the EGR valve as I run the RPM's up and down with the engine warmed up and it appears to be moving about a 1/4 of and inch maybe? Is the EGR serviceable?
 

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OK, got a new intake manifold gasket and will go on a vacuum leak hunt tomorrow with manual in hand. Will test and check anything vacuum, will let you know the outcome.
Thanks!
 

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Well, not a vacuum leak, even disconnected the power brake booster and capped the hole, no difference. Tested every component the book says to test, all perfect. Coils test fine as well. Sending in the ECM to SIA this week, got to rule that out.

Got a new question, why does the butterflies inside the intake sound like a bad lifter when I increase the RPM?
 

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Could it be old fuel?
Bad fuel pressure? --> bad pump or clogged filter
Bad or clogged fuel injector(s)?
Does the misfire isolated to a cylinder(s)?

Does it have good compression? That being said I would be shocked if a car with such low mileage had an issue there.

The butterflies in the intake plenum are driven by an electric motor with a spring loaded worm gear that often brakes. From what I understand, do not remove the motor as you will have to recoil the spring. I can't speak fully for a first gen as I have a second gen but you should be able to test the motor with these steps:
  1. Get a 3v battery or 3v power source if you have access to one. I used 2 1.5v watch batteries in series.
  2. Disconnect the electrical connection for the motor.
  3. Take off the airbox assembly
  4. With the engine off, Use jumper wires from the battery to the motors pins to test it.
  5. You basically run the motor until it hits it's stop, then use your hand to open the main throttle butterfly to peak in there and see if they are working
  6. When you switch the polarity of the jumper wires the motor will soon the opposite direction, then check there position again.
The butterflies should turn 90 degrees just like the main butterfly. If the motor spins but the butterflies don't move then you got issues and I would recommend purchasing a working plenum.

My butterfly motor also ticks but it seems to be working fine

I have a 2nd gen so you should consult the service manual for your car. There should be one you can download on 3sx.com

Hope that helps!

30k miles! Lucky you!
 

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Discussion Starter #11
Drained the tank and put in new fuel, compression on # 3 138 psi, all the rest 144, motor cold. Did a leak down for the heck of it and results were 90 to 93 percent. Fuel filter is original, have a new one on order. Can't really figure out how to check the fuel pressure without cutting a line. Rigged up a way to check the fuel regulator with air and it is within tolerances of the book.
If I put a continuity tester on terminals 1 and 4 of the TPS, (I stick sewing needles in the wire) when I move the throttle and right where it loses the continuity, that's where it starts to miss. It idles just fine even when I turn the AC on and the idle increases or first thing when its cold, but start rolling the throttle and the miss is there. If I roll it on real fast it appears to take it fine but when I roll it on slowly it will miss until I quit and let it return to idle. It doesn't appear to be any one cylinder, its kinda all over the place. Injectors look clean but I really have no way of testing them under pressure. That's a funny thing, if you look at the plugs, they all look the same and are burning fine????

My intake has an allen bolt on the throttle body side you can watch move when you test the motor.
When I change out the fuel filter I,m going to see if I can't rig something to test the fuel pump pressure.

Thanks for the reply.
 

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Jeez I bet right about now you wish you had a carburetor & distributor 🤣 I'm not a professional mechanic but it sure sounds like an ignition timing issue. As you mentions before the timing light was all over the place.

Here are some thoughts:
What if the ECU & coils are working just fine and it is just getting bad data from the Crack Position Sensor?
What if you are not getting enough juice to the coils due to a bad wire or a bad Battery/Alternator?

Where are you based out of?
 

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Discussion Starter #13
Yep, all this stuff is crazy...….. It would be better if it would just stop running......
The Tach was working perfectly so I didn't put much thought to it but it's a easy check when I get the ECM back if needed.
I'm in Riverside Ca.
 
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