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Will a bad power transistor still allow the car to idle fine?

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Gregory

15+ Year Contributor
296
1
Aug 22, 2005
Malvern, Pennsylvania
Ok, sorry to make a new thread. I did search for this.

My car will misfire at higher RPM's (5k+), but it idles and drives fine in normal driving conditions. I've check everything (plugs, wires, coils, boost leak, compression, TPS), but the power transistor. I did the procedure today to check it, and it seems that it is bad (for both sets of cylinders) according to the test. This confused me though since the car drives perfect untill you start to go heavy on the throttle. Does this make sense? Should I invest in a new transistor?

I'm confused because it seems like everyone who has problems with their transistors has idling problems and not a problem like this, or at least, not that I could find.

What are your thoughts? Can a power transistor cause a misfire only under heavy load at high RPMs?

Thanks,
Greg
 
1st... what are these "Tests" that you did :hmm:

I doubt its the powertransistor and would focus more on plugs/wires/fuel...

How did you check the plugs/wires??

I checked it using this procedure:

http://www.dsmtuners.com/forums/articles-electrical-wiring/229419-how-test-power-transistors.html

I've replaced the plugs and I have COP ignition. I got COP when this problem started and I thought it was bad coil packs. Anyways, I'm pretty confident its not coils or plugs.

I'm having the same issuses!!! at full boost around 5k it sputters but nothing else...its crazy!! gonna switch my power transistor tonight and see if that fixes it

Well, please let me know how this turns out because if it helps you, I'll be optimistic. And, does your car idle/drive fine as well?


To anyone else: I'm still looking for a definitive answer whether it's possible or not that the transistor would cause this. Thanks!



Well I just found this:

IGNITION MODULE

The electronic control for the ignition system. The module receives a signal from a magnetic pickup or Hall effect sensor in the distributor. The module uses this signal to open and close the ground circuit to the ignition coil to fire the spark plugs. The ignition module itself may be located inside the distributor (GM and some imports), on the distributor housing (some Ford applications) or in the engine compartment. Some modules also control timing advance and retard. If a module goes bad, it usually goes completely dead. The engine won't run because there's no trigger voltage to the ignition coil.

So I'm guessing that means my transistor is fine.
 
When mine went It would only run on two cylinders. The other two were dead. Replaced the coil pack and that didn't fix it so I replaced the "power transistor", that fixed it.

You know that has happened to me before and It was driving me crazy. It would buck and misfire around 5K only under boost. I replaced the plug wires with the cheap Autozone ones and it didn't fix it. I came full circle in my trouble shooting back to the plug wires after replacing or swapping, Plugs, coil, BLT. So I bought the expensive MSD wires from NAPA ($75) and it friggin fixed it. Just a thought.
 
Last edited by a moderator:
You know that has happened to me before and It was driving me crazy. It would buck and misfire around 5K only under boost. I replaced the plug wires with the cheap Autozone ones and it didn't fix it. I came full circle in my trouble shooting back to the plug wires after replacing or swapping, Plugs, coil, BLT. So I bought the expensive MSD wires from NAPA ($75) and it friggin fixed it. Just a thought.

Well its definitely running on all four cylinders most of the time. That's the weird thing. It makes me think it's not the power transistor.

I would try changing plugs, but I eliminated the plug wires with coil-on-plug. So idk what to do.
 
Well its definitely running on all four cylinders most of the time. That's the weird thing. It makes me think it's not the power transistor.

I would try changing plugs, but I eliminated the plug wires with coil-on-plug. So idk what to do.

Do you have an ignition amplifier to go along with the COP??
 
No I don't. I didn't think that's necessary. Do you know otherwise?

Plus, the problem was present before I put on COP.

Well typically the cop wont get a good enough spark without one. And if you search you will see that it is usually reccomended not to run a cop without some type of ignition amplifier being an MSD DIS2 or some other type. And from my own experience with one I would highly reccomend an ignition amplifier. I had problems with my cop cutting out anywhere above 4k rpms when i didn't have one.
 
Well typically the cop wont get a good enough spark without one. And if you search you will see that it is usually reccomended not to run a cop without some type of ignition amplifier being an MSD DIS2 or some other type. And from my own experience with one I would highly reccomend an ignition amplifier. I had problems with my cop cutting out anywhere above 4k rpms when i didn't have one.

I'll look into this tomorrow. I have to get going for the night. Real quick though, is it difficult installing an amplifier? And how much do they cost?
 
I'll look into this tomorrow. I have to get going for the night. Real quick though, is it difficult installing an amplifier? And how much do they cost?

Look around in the classifieds for one. Other wise expect to pay 500ish for some of them.. And they are very easy to install.
 
Just a thought here, but maybe your PT works fine at idle because of the lower temps? I know heat is a leading cause of these to malfunction. mine would work fine tell operating temps then start to mess up. Also is your PT grounded through the case?
 
My car ran intermittently on 2 cylinders a while back. Tried different coil packs first, no change. Turned out to be the connector on the wiring harness to the power transistor. The female parts of the connector had just lost tension from age. We bent them a little bit with some spring wire and it totally fixed the problem. With my car though the problem wasn't just at high power, it was also at idle. Wish there was a way to buy all these weird OEM connectors brand new.
 
LOL thats for brand new :) i got mine for 125 shipped...it was used a bit but still :) just look around for it you will find one for cheap

I just saw a couple in the classifieds. What all do I need to go with it? Do I need a tach adapter? A couple people referenced those in the forums.

What plugs are you using and how much boost you running?

Running BPR7es now gapped at .028. Running about 18lbs of boost, but the problem continues no matter how little boost I run.

Just a thought here, but maybe your PT works fine at idle because of the lower temps? I know heat is a leading cause of these to malfunction. mine would work fine tell operating temps then start to mess up. Also is your PT grounded through the case?

It seems a little too consistent to think that its heating up and cooling off exactly when I go in and out of that RPM range. Interesting thought though.

I don't have it grounded through the bracket if that's what you mean. I thought it didn't matter though. I believe it's grounded through the harness.

My car ran intermittently on 2 cylinders a while back. Tried different coil packs first, no change. Turned out to be the connector on the wiring harness to the power transistor. The female parts of the connector had just lost tension from age. We bent them a little bit with some spring wire and it totally fixed the problem. With my car though the problem wasn't just at high power, it was also at idle. Wish there was a way to buy all these weird OEM connectors brand new.

I'll take a look at the connectors, but I see you say you had problems at idle too. Seems like a bad PT always causes a problem with idle, but mine idles fine.


Thanks for all the responses guys! I might try a MSD DIS2. I don't know much about them though.
 
Yes you will need the tach adaptor for it also. Otherwise you wont see the rpms at all. Sucks for tuning haha....Did you happen to check for vacum leaks?
 
Damn that sucks. I guess I'll be keeping my eyes out for one. I've done lots of boost and vacuum leak checks in the past, but it wouldn't hurt for me to do another... Maybe I will tomorrow.
 
How did it feel like running on two cylinders? And did you have a power surge in first?

Running on 2 cylinders like this (power transistor problem) is absolutely horrible. The engine will barely run.
There is no surge associated with it, unless it cuts in and out, intermittent problem, like it could be if the problem is in the electrical connector.
 
Running on 2 cylinders like this (power transistor problem) is absolutely horrible. The engine will barely run.
There is no surge associated with it, unless it cuts in and out, intermittent problem, like it could be if the problem is in the electrical connector.

Okay then damn that’s not my problem I’m having. I tried my spark plugs and I just order ngk wires I’m hoping that fixes it. But my car idles perfectly fine and revs fine. When I put it in first it had a power surge and wants to take off, but no matter 1-3rd gear it won’t let me go past 35 without bogging horribly and popping. Do you possibly have an idea?
 
When did you last check for a boost leak?

FYI: you're reviving a 9-year-old thread. The original posters are not likely to respond to this at all.
 
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