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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #1
I’m having trouble diagnosing my 91 VR4. It was a stock car, and neglected until I got it at 225k, 230k now.

Idles fine, when pulling away from a stop it randomly pops in the exhaust, a good amount.
but runs fine from (about)2000 to 3500 RPM, sometimes 4000….
Between 3500 and 4000rpm it pops rapidly like its hitting a rev limiter. Usually unable to go over 4000... Sometimes it does, but Its always feels low on power, AND its temperature sensitive… it likes the cold.

But here’s what I’ve done:
Cleaned MAF
To the manual I adjusted the idle screw CAS and TPS (tested CAS and TPS with multimeter)
New coil/plugs/wires/ignitor (tested all)
Tried 3 computers, recapped two of them myself.
Replaced the vast majority of vacuum lines
New fuel pressure regulator
Bypassed the fuel pump resistor to run at a full 12v with both high and low relays.

Less scientific, I drove around with an aftermarket boost gauge…. intake system seems to be spot on without leaks hitting 7lbs in perfect sync with the guage on the dash. As well as sprayed starter fluid all over and didn’t find vaccum leaks.

Since replacing the ECU’s coolant sensor the check engine light is off, no codes.
 

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How did you clean the MAF? Because most MAF cleaners will damage the style of MAF these cars use.
 

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If the car was neglected, my guess would be clogged injectors.
I would pull them and send them out to be cleaned/flow tested.

Just my .02

Bob.
 

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If you pull off the plenum you can follow the vacuum lines to the wastegate actuators, replace those. I had a similar problem to yours and that ended up being the culprit after a long time chasing the problem. What happened is the turbos overboosted between 3500 and 4000 rpm (aftermarket gauge was showing 20+ psi) resulting in too much engine load and ultimately fuel cut. If memory serves the boost is controlled by the ECU for engine LOAD not boost psi. Replacing those vacuum lines allowed the ECU to control boost again and problem solved. After that I went with an aftermarket electronic boost controller and ran new lines to the wastegate actuator.
 

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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #5
How did you clean the MAF? Because most MAF cleaners will damage the style of MAF these cars use.
CRC carb cleaner "safe for MAF sensors" or so the can says.

took the airbox off and sprayed the crap out of it, let it dry, reinstalled.
 

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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #6
If the car was neglected, my guess would be clogged injectors.
I would pull them and send them out to be cleaned/flow tested.

Just my .02

Bob.
I'll call around see what shops offer that.
 

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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #7
If you pull off the plenum you can follow the vacuum lines to the wastegate actuators, replace those. I had a similar problem to yours and that ended up being the culprit after a long time chasing the problem. What happened is the turbos overboosted between 3500 and 4000 rpm (aftermarket gauge was showing 20+ psi) resulting in too much engine load and ultimately fuel cut. If memory serves the boost is controlled by the ECU for engine LOAD not boost psi. Replacing those vacuum lines allowed the ECU to control boost again and problem solved. After that I went with an aftermarket electronic boost controller and ran new lines to the wastegate actuator.
Your post seems the most helpful so far...

Im not yet familiar with how these cars control everything and what data is sent to the computer...

But I do know that sometimes the dash shows 14ish PSI when stock it should only do 7

I wonder if thats related. (if the ECU cant control the wastegates correctly my A/F will be way off.


do you have a diagram of the wastegate lines? or know how to test the actuators?
 

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The stock boost gauge is NOT very accurate (in fact it is pretty useless).
It uses some info fed to the ECU to "show" boost.
You said you added an aftermarket gauge, exactly where did you tie it into the system?

We can all guess at what your problem is, but until you are able to log data and post up the results, it's just guesses....

Bob.
 

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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #10
you may want to try a proper boost leak test and pay close attention to the throttle shaft seals
I'll need order what I needs to do the test.

I also just picked up a new in box CXracing front mount intercooler kit with all new gaskets an hard pipes.
Ill install that to see if it makes a difference

not the kit I wanted, but for the price I couldn't pass it up! ($400)
(edit typos)
 

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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #11
The stock boost gauge is NOT very accurate (in fact it is pretty useless).
It uses some info fed to the ECU to "show" boost.
You said you added an aftermarket gauge, exactly where did you tie it into the system?

We can all guess at what your problem is, but until you are able to log data and post up the results, it's just guesses....

Bob.
So its not accurate huh? I didnt think so... It often shows a mad sweep from vacuum to 7 PSI sometimes as high as 14...

I had my harbor freight "nitro boost gauge" it seemed to read in sync with the dash gauge, but that day it never showed 14 on the dash.... It would have been interested to see if they diverged.

I had a vacuum "T" on the larger nipple on the intake, about an inch behind the throttle body.

I know its harbor frieght... yuck, it cheapest I could find to do a haha ebay turbo ninja riding lawnmower Im building... but since it was in the garage I used it.
 

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CRC carb cleaner "safe for MAF sensors" or so the can says.

took the airbox off and sprayed the crap out of it, let it dry, reinstalled.
If you have the can it'll say "Do not use on vehicles with a Karman Vortex air flow sensor (e.g. Toyota Supra Turbos, Mitsubishis and most Lexus vehicles)". I know this because that quote is on the CRC page for their MAF cleaner.

Also good luck with your CXracing FMIC. Mine looks cool, and works well, but the fitment on the piping is awful. Also be prepared to have to deal with cooling issues as a result of the FMIC blocking the radiator.
 

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Since no one has mentioned it I’ll toss this out there- that sounds a little like either spark leak (as in faulty plug wires or ceramic isolators) or spark blow out due to cooler temperatures. Not sure if the temp has dropped off in your area, but some times the additional density plus a well worn plug can cause it. It’s easy to pull a front plug or 3 and check the condition as well as the plug gap to make sure everything is still within spec. If they look questionable it may not be a bad idea to toss a set of plugs in it and see how that goes. As for the wires, I’d just pop the hood at night with no residual light around and look for the laser light show...😂 You’ll see pretty quickly if there’s a problem with them.
William-
 

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Your post seems the most helpful so far...

Im not yet familiar with how these cars control everything and what data is sent to the computer...

But I do know that sometimes the dash shows 14ish PSI when stock it should only do 7

I wonder if thats related. (if the ECU cant control the wastegates correctly my A/F will be way off.


do you have a diagram of the wastegate lines? or know how to test the actuators?
Sorry it has been so long. Check out www.chromedecu.org for more info on how the ECU controls things. Basically the wastegate spring (stock) is about 7psi and there is a vacuum/boost line going to each wastegate actuator. The boost is bled off by a solenoid valve on the firewall to control what pressure the wastegate actuator sees allowing for over 7psi of boost. If that line is broken or the solenoid valve prevents pressure from being seen at the wastegate you will overboost because the wastegate will never open.
I do not have a vacuum diagram, but if you look for each waste gate actuator you should see a rubber line coming off of it, replace that and keep following it back and replacing it until you are complete. That should help, assuming this is the problem.
 

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You find the solution to this?

My dash gauge would do a big sweep all the way up with partial throttle and lack power studdering horribly.

The coupler on my y-pipe kept popping off and would leak only while in boost. Vacuum the car ran fine but as soon as it boosted it'd start leaking boost but registering load and increasing fuel when there wasn’t any air.

Torqued the couplers on the aftermarket Y-pipe extremely tight and did the tennis ball mod on the intake and have had zero issues since.
 

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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #16
You find the solution to this?

My dash gauge would do a big sweep all the way up with partial throttle and lack power studdering horribly.

The coupler on my y-pipe kept popping off and would leak only while in boost. Vacuum the car ran fine but as soon as it boosted it'd start leaking boost but registering load and increasing fuel when there wasn’t any air.

Torqued the couplers on the aftermarket Y-pipe extremely tight and did the tennis ball mod on the intake and have had zero issues since.
So far I found that half the "advice" given seems to be from a bunch of keyboard mechanics who dont have the slightest clue.

I did order this back in may when I found the previous owner had used masking tape (not even duct tape!!) to make a "gasket" for the stock Y pipe...
it sat in my garage for a month until I was getting ready to go to a car meet.... the stock pipe blew off on my way to the car wash.

It ran better after installing that, but still not running perfect, I know I have more leaks and every day I work on it I find another and things get better.
unfortunately its hard finding the leaks... and I have a lot of projects (also doing a 70 firebird, 03 marauder, 02 ZX, 01 ZX12) while running two businesses.
 

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Sorry it has been so long. Check out www.chromedecu.org for more info on how the ECU controls things. Basically the wastegate spring (stock) is about 7psi and there is a vacuum/boost line going to each wastegate actuator. The boost is bled off by a solenoid valve on the firewall to control what pressure the wastegate actuator sees allowing for over 7psi of boost. If that line is broken or the solenoid valve prevents pressure from being seen at the wastegate you will overboost because the wastegate will never open.
I do not have a vacuum diagram, but if you look for each waste gate actuator you should see a rubber line coming off of it, replace that and keep following it back and replacing it until you are complete. That should help, assuming this is the problem.
The boost gauge is an output from the ecu based on inputs from airflow and RPM.
When there's a leak after the turbo, like at the y pipe, the airflow goes right up, as does the boost on the gauge.

Fix the leaks?

The other place for a leak is the rear turbo o ring.

Then fix the head gaskets.
 

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The boost gauge is an output from the ecu based on inputs from airflow and RPM.
When there's a leak after the turbo, like at the y pipe, the airflow goes right up, as does the boost on the gauge.

Fix the leaks?

The other place for a leak is the rear turbo o ring.

Then fix the head gaskets.
Where is my quote did I talk about a boost gauge?
 

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91 VR4 wish scissors BBK wheels and tint
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Discussion Starter #19
The boost gauge is an output from the ecu based on inputs from airflow and RPM.
When there's a leak after the turbo, like at the y pipe, the airflow goes right up, as does the boost on the gauge.

Fix the leaks?

The other place for a leak is the rear turbo o ring.

Then fix the head gaskets.
Im not sure why you said head gasket...

BUT the comment about the airflow vs RPM calculating to the boost gauge.

If I understand correctly this explains a ton and will help with diagnosing.

I have a new Oring from cherry hill (though I'm starting to understand why the previous owner had a fetish with RTV.)
 
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