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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Dodge
Satisfied Customers: 1526
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I have been going around in circles trying to diagnose a

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Juan, I have been going around in circles trying to diagnose a 1994 Dodge Stealth V6 (Mitsubishi). Engine idles perfect and runs perfect at cruising speed. But off idle, if you give it pedal or do a snap throttle, it bogs and dies. I have scoped the MAF, the TPS and has a new IAC motor. Also, it does not do it when dead cold. Any ideas?

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

Have you had the fuel pressure tested at all to see if fuel pressure is an issue or were any fuel components replaced at all?

Let me know so we can continue,thanks Pete.

Customer: replied 1 year ago.

New fuel filter. Pressure is 42 psi at idle and 52 psi on snap throttle.

Is this 52 psi when the fault occurs as well?Does the pressure maintain on a pressure gauge with the key off?

Hi there. sorry it took me a little while to respond.

It sounds like the issue happens after the system goes into Closed Loop. This is a very finicky MAF sensor and it might not be sending the right signal to the ECU. Can you scope injector signal and see if ON time increases along with throttle opening?

Ah, almost forgot... scope signal frequency from MAF to ECU as well; it should increase proportionally with RPM.

Customer: replied 1 year ago.

Here is a little more information. When the car came in the first time, it had a bog but wouldn't die every time. In trying to pinpoint the problem, I found that by disconnecting either the MAF or the IAC, the bog totally went away and made no difference to the running. I eventually discovered that 3 of the 4 stepper coils in the IAC were bad and I replaced the IAC. Engine ran perfect for a day. Then the bogging started and is worse. I can no longer get it to stop by disconnecting either MAF or IAC??? Also, to complicate the issue, this is a 1994 but only has an OBD II DLC??

I can't see how the IAC would influence acceleration - unless it was robbing current from the system. I'm leaning more towards believing that ECU is not getting the right signals from the MAF, which is causing it not to open the injectors longer or advancing the ignition timing proportionally.

The '94s had the hybrid set-up and need a special adapter cable to connect to the scan tool for data display. However, two-digit codes can be retrieved by grounding pin #1 of the DLC and counting the check engine light flashes

Customer: replied 1 year ago.

As far as data display, the adapter doesn't work as it doesn't have the old style DLC. Just the OBD II. I have the adapter but nowhere on this car to plug it in. I'll do the scope tests and let you know.

Thinking back now, the strategy is such that if the MAF fails, the ECU will look at TPS and CKP signals to determine fuel injection and ignition timing. So, how about checking both of those signals and seeing what's going on with them?

I meant an adapter for the old scan tool to plug in to the 16-pin DLC. Regardless, you can still ground pin #1 of the 16-pin DLC to get flash codes.

Customer: replied 1 year ago.

Hi Juan, my schematic must be wrong. I'm getting a square wave signal from the MAF the fluctuates from 1 to 5 VDC on the voltage scale. Would you have the signal wire colors for the MAF, TPS, CKP that I need to scope? Thank you. Tom

I think you've found part of the problem: the MAF does put out a square wave-type signal. However, amplitude/voltage should stay fixed with time/frequency increasing in direct proportion to RPM - in this case, a fluctuating amplitude indicates a problem with that circuit.

Signal wire colors:

MAF - Blue/Yellow from ECM pin 70 to MAF connector pin 3

TPS - Brown/Red from ECM pin 64 to TPS connector pin 2

CKP - Black/White from ECM pin 69 to Distributor connector pin 2

CMP - Red/Yellow from ECM pin 68 to Distributor connector pin 1

Customer: replied 1 year ago.

Hi Juan, I'm a little confused here. If the Voltage does not vary, how can we have a square wave using voltage? When testing frequency the idle frequency is about 40 Hz and climbs steadily to about 700+ Hz. On snap throttle, It spikes to about 700+Hz and dies. By the way, before I changed the IAC motor, I did have a brand new MAF and TPS installed with no change.

I do not understand your question. The square wave is the result of the circuit being switched on/off abruptly by the trigger; how long and how often the switching occurs determines the frequency. How much voltage is applied to the circuit determines the signal amplitude, which should remain at a steady value throughout the signal cycle (see attached graphic).

Is this the MAF signal reading, "On snap throttle, It spikes to about 700+Hz and dies"? If so, it looks like the MAF is OK.

Remember the ECM will look at CKP and TPS signals when the MAF is disconnected, since RPM is OK on a steady rise, I would look at CKP signal next.

Customer: replied 1 year ago.

Yes, that is the exact square wave I am getting whne measuring voltage. Currently all sensors are connected, but still getting the same result. I will scope the CKP and CMP in the morning.

The optical distributors Mitsubishi put on these cars are also very finicky and this particular one being almost 23 years old certainly doesn't make things easier for you.

Set your scope first, then monitor the screen while a helper turns the ignition on - a sure sign of trouble is when the CKP outputs a pulse signal when the ignition switch is first turned on (engine not cranking).

Customer: replied 1 year ago.

Sorry for the delay. I scoped the CKP and CMP. Both are putting out nice square wave signals. The CKP has a square wave 0 to 5 voltdc. The CMP square wave is also from 0 to 5vdc. On snap throttle they stay at the same wave just increased frequency. I really don't have much hair left :-) No pulse from the CKP whne the key is just turned on.

Check the TPS - the signal should rise smoothly from less than 1 to about 4.9 volts. If TPS checks out OK, then check injector signal - on-time should increase proportionally with throttle opening. Check for different signal with fuel return hose pinched off - if shorter (less on-time), check fuel pump volume.

Customer: replied 1 year ago.

TPS voltage goes from .64 to almost 5.0 VDC and no fall outs. Injector on time is 3ms at idle and 12 ms on snap throttle. Now here is something interesting that I just found. With KOEO if I open the throttle. the low speed fan relay gets energized as does the purge control solenoid valve. If I shut the throttle, they stop. Even stranger, if I do not touch the throttle, but disconnect the TPS, the fan comes on and the purge control solenoid stays on steady. This is all with KOEO.

That's peculiar indeed. Let me see what I can dig out on that subject. Be back in a bit...

By the way, I've been working under the assumption this a 1994 Dodge Stealth with the base 3.0L SOHC tuned to meet Federal Emissions. Can you confirm that?

Well, nothing out of the ordinary came up as a result of my search.

Go ahead and disconnect the engine temp sensor, then take the car out and drive it hard to see if it responds differently. If it doesn't bog down, then you'll need to make sure the temp signal voltage decreases inversely proportional to engine temperature. If it does, then I'd say it's time to try a new ECM.

Customer: replied 1 year ago.

I'll try the temp sensor, although the scanner is reporting the proper engine temp. I already have tried another ECM.

Looking back on the posts, it seems you did not report on what happened with the fuel return line pinched off. Did you try that?

Customer: replied 1 year ago.

First of all, I would like to thank you for sticking with me to solve this problem. No difference with the fuel return line blocked. Also, whne looking at fuel pressure, as soon as I do a snap throttle, the pressure goes to about 52 psi, so the regulator seems fine. I tried to run with the ECT sensor disconnected and same problem. I replaced the ECT sensor for good measure and still the same thing. The only way I can getthe bogging and stalling to stop is to raise the idle RPM to about 1400 RPM. Then there is absolutely no bog or stalling.

Raising the idle changes TPS setting, which also raises its voltage; which leads me to believe that's where the issue lies. Can your scope capture live signals and store them for later display? If so, can you capture a few snap-throttle screens with the engine running both cold and warm and upload them for me to see?

Customer: replied 1 year ago.

I would have thought so too except that when I scoped the TPS it rises form around .60 to almost 5 volts without any glitches. I am away from that vehicle until Tuesday andwill scope the live signals and upload to you. Here are the shots of the scope. KOEO-COLD, KOER-COLD, KOER-HOT.

Customer: replied 1 year ago.

Hello Juan, did you get the screen shots of the TPS scope images? Thank you. Tom

Saw the screens. I would have liked to have seen a much faster time setting and a slower throttle application that could have potentially revealed any glitches. As presented, I cannot see anything that would tell me there's something wrong with that signal.

We know that in order for this engine to accelerate, its control module needs to see increased throttle opening and greater air volume signals for it to increase injector on time (fuel pressure and volume should increase by virtue of no vacuum to the pressure regulator) as well as ignition dwell and timing.

So, the issue here is to find what's askew; input or output? Can you rig an external fuel source you could activate while accelerating under load? I'm thinking something like a can of choke cleaner and a long hose that can be "T'd" to an intake manifold vacuum port. Let's try that and see what happens - other than that, I'm afraid I've run out of suggestions...

Customer: replied 1 year ago.

I can do that, but please remember that it only stalls and bogs on initial acceleration. If I ease into the throttle very slowly, everything is fine, even under load. One other thing I found today - you know how all those relays were activating with KOEO (Fan, purge solenoid etc.) ? Well, if I pull the MFI Relay connector everything stops activating

Do you mean the engine fan will come on and stay on KOEO even when the engine is cold?

Customer: replied 1 year ago.

Yes. Please see my post of

Wednesday, August 31, 2016 7:18 PM EST

If that's the case, then we're back at engine temp signal as either being bad coming in or not processed correctly by the ECM - I don't see any other reason why the ECM would turn on those relays/solenoids.

Customer: replied 1 year ago.

from what I can see, there are two temp sensors. One for the warning light and one for the computer. The one for the ECM is new and has a resistance of 1.7 kOhms. When hooked up to the scanner, the temperature climbs steadily to about 206 - 210 and stays in that range. Is there a possibility that the MFI relay is shorting and causing these issues? Also, whne I hook up to my gas analyzer, the HC, CO, CO2 and O2 are perfectly within spec. Whne I do a snap throttle, the HC's climb past 1200 and the CO to 1.6.

I'm having problems with my database again, but if I remember correctly, the MFI relay is the source of ignition voltage for all computerized circuits - a short in the MFI switch circuit would mean there would be power to everything all the time regardless of whether the ignition is on or not; a power on the control circuit would mean there would not be power at all, plus there would be a blown fuse.

Customer: replied 1 year ago.

Would you have the circuit tests to see if that is the problem. I don't have full schematics. Also, I am not sure I understand what you mean here "a power on the control circuit would mean there would not be power at all, plus there would be a blown fuse."

Sorry, I meant to type "a short on the control circuit...". The point I was trying to make is that I can't see how the MFI relay would cause the bog down - I'm attaching the circuit diagram for you to review.

Customer: replied 1 year ago.

I think one of the reasons I am having so much trouble is that I am trying to hit a moving target. In other words, every time I start this engine, I get some different behaviour. This morning, I let it warm up and out of desperation, started to disconnect different things and tried snap throttles. Lo and behold, when I disconnected the IAC motor, I could no longer get it to bog. Shut it down to answer the phone and when I restarted 2 minutes later it was bogging again. Now normally an engine needs air, fuel and spark. The following are acting exactly as they should be on the scope. TPS, MAF, Injectors, Spark and fuel. Timing also jumps to about 30 degrees BTDC on the snap throttle. Injector on time jumps from 2.7 to 12 ms. The one thing I have not ruled out is the distributor, but not likely. Is there any way that this site allows a phone conversation at extra cost?

The site does offer phone chats at an additional cost. Problem is I'm based in the Dominican Republic, where I retired to some time ago, and phone service in this country is spotty at best. We mostly use the web to communicate with friends and family back in the States.

Now that you mention the distributor, have you tried setting basic timing couple of degrees advanced? If you haven't, do you need the procedure for that?

The site does offer phone chats at an additional cost. Problem is I'm based in the Dominican Republic, where I retired to some time ago, and phone service in this country is spotty at best. We mostly use the web to communicate with friends and family back in the States.

Now that you mention the distributor, have you tried setting basic timing couple of degrees advanced? If you haven't, do you need the procedure for that?

Customer: replied 1 year ago.

Yes, the procedure would really help. As well as setting the TPS as I had to eyeball a mark.

OK. That will take me some time since I'll have to transfer pics and text to a Word doc then convert it to PDF for uploading (and only God knows how much I struggle with those programs!). If it's OK with you, I'd rather tackle that task tomorrow.

Friend came by and gave me a hand. See both procedures attached.

Let me know if that does it.

Customer: replied 1 year ago.

Good day Juan. I understand what you mean about programs. I'm an electronic engineer and wrote thousands of programs before retiring and getting all my mechanics certs. Usually vehicle electronics are easy for me, but not on this beast. Never had one like this. If you ever need a hand with programs, I would be happy to help. As far as setting the initial timing, I have looked high and low on the balancer, but there is no timing mark. The scale is present on the front cover, just no corresponding mark on the balancer.

Thank you for offering to help with the programs - my issue is more of an age thing ;-(

back to engine timing, there is always the old fashioned option of removing bank 1 valve cover and cylinder 1 spark plug to find TDC - cam lobes flat and piston all the way up...

Your mention of vehicle electronics made me realize I've been making suggestions under the assumption that mechanical/valve timing is set to specs. Is it?

Customer: replied 1 year ago.

Well, I can't confirm as I did not do any internal engine work. Since it sometimes runs perfect, and idles flawless, I have to assume that the valve timing is good.

OK. Looking back at some of the past test results I see HC was climbing past 1200 ppm which would support your theory of ignition timing and dwell not catching up to air/fuel intake. How about taking dwell readings at idle and then at various throttle openings to see if those increases are as proportional as those as injector on-time - chance to play with those two-channels!

Customer: replied 1 year ago.

Dwell as in coil burn time?

Yes - although I've never heard someone call it that before now...

Customer: replied 1 year ago.

Lol. Leave it to me to come up with it. I will scope this in the morning. Thank you.

I'll be back online for a bit tomorrow afternoon/evening, then I'm off on a trip back to the US to visit friends and family. I'll try sneaking a login when my wife is not looking... ;-)

Customer: replied 1 year ago.

Enjoy your trip. What part of the US? I have to catch up on some backlog today as my other customers are getting impatient. I will come in tomorrow and just work on the Stealth. Again, Juan, thank you for sticking with me on this problem. Tom

Going to Richmond, VA. Be back in a few days.

Best Regards.

Juan Crespo, ASE Certified Technician
Category: Dodge
Satisfied Customers: 1526
Experience: A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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