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Thread: Diagnose this mystery

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  1. #1 Default Diagnose this mystery 
    OK, so this past weekend I was in NJ helping another team at a lemons race. One of their cars was having issues and we couldn't figure it out.

    Car: Triumph TR7 with a buick 3800 supercharged engine.

    Issue: The car refused to rev past ~3500rpm.

    Longer issue: The issue seemed to coincide with when boost came on. as soon as the gauge rose past 0 to 1psi pressure it started missing, stumbling, hesitating, whatever you want to call it. It would not rev any higher.

    However, if you disconnect the MAF sensor and essentially throw it into limp mode it would rev just fine. It just wouldn't build boost because the computer was keeping the bypass open the whole time. If you removed vacuum from the bypass sensor/actuator you could build about 5psi. Car still went like hell in limp mode.


    My first diagnosis: Ignition. Something was cutting the engine off early when boost built. They have a big aftermarket tach in the car that runs off one of the coil trigger wires. When you hit ~3500 it goes nuts jumping around all over the place. Just like it would if the coils were being fed bad signals. In fact I had this exact problem with the daytona where the distributor pickup ring was loose and sending crappy signals at higher RPM.

    What was replaced:
    1. Crank Position sensor
    2. Cam Position Sensor
    3. MAF sensor
    4. MAP sensor
    5. Supercharger bypass actuator/solenoid thing
    6. Coil packs
    7. Ignition Module
    8. Fuel pressure regulator
    9. O2 sensor

    Each of those was replaced with either a new part or a known good part. The problem continued.

    Other details:
    The car throws no codes. None. no misfire codes, nothing. Plugging in a professional scan tool showed nothing out of the ordinary.

    The car was running rich, but i'm not sure that's related, It seems to always run rich.

    The car has a modified ECU to write out a lot of extraneous things it doesn't need. (think AC, Door sensors, impact sensors, etc)

    The car ran perfectly at NH last year. The only thing they changed between then and NJ was the coil packs to new ones. Swapping those back was the first thing we tried.


    So. can you figure it out? We couldn't. I spent a solid 8 hours throwing ideas at that damn car. It has to be something simple, and i just couldn't find it.
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  2. #2 Default  
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    Maybe it's not a problem cased by revs but by throttle position. Is the throttle position sensor functioning properly / tightened in place / adjusted?

    You've probably already done it, but it's about the only sensor you haven't mentioned already.

    This is for after you've already checked everything.... check the fuses, not just for continuity across the blades but for a solid connection with the fuse holder. My 240sx would not start in the cold. I had tried everything. My final solution - just don't let the car get cold if I plan to drive it. Next owner found the problem - a good fuse with a bad connection in the fuse box.

    Is there any way the engine could 'rock' at higher rpms and pull on electrical connector or ground?

    -Justin
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  3. #3 Default  
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    BTW, this may be the biggest hail mary in the history of diagnostics.

    Asking people to diagnose a British car with a turbo Buick engine from 500 miles away.

    -Justin
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  4. #4 Default  
    Oh, race weekend is over. It's just bothering me still.


    We talked about grounds, they need to all be checked. Throttle position sensor works fine, we checked that with the scan tool. We went through the fuse box, but no idea how thoroughly, I wasn't the one that did it.

    We didn't check any knock sensors. I guess it could be that going bad and pulling back the engine.
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  5. #5 Default  
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    Two totally unrelated things to look at:

    1. Was the TPS installed correctly? The TPS has tabs that grip on a pin on the end of the throttle shaft, it needs to be canted and "latched" when it's installed, not just plopped on. This would actually make sense that the car is allowed to rev with the MAF unhooked at the ECU should use a different input hierarchy with something broken.

    2. What is the transmission setup, is it a custom swapped manual? Could it be possible a neutral switch or clutch switch is broken, forcing the car to think it's in "park mode" and engage the park/neutral rev limiter? The tach jumping around could be a sign of a deliberate ignition cut rather than bad signals.
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  6. #6 Default  
    I agree it's a deliberate ignition cut, it looks exactly like that. We just can't find the source.

    TPS is fine. It's never been removed and we tested and confirmed it's working.

    I forget what trans it has, but anything related to an auto has been coded out in the ECU.
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  7. #7 Default  
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    Quote Originally Posted by cegan09 View Post
    Oh, race weekend is over. It's just bothering me still.

    We talked about grounds, they need to all be checked. Throttle position sensor works fine, we checked that with the scan tool. We went through the fuse box, but no idea how thoroughly, I wasn't the one that did it.

    We didn't check any knock sensors. I guess it could be that going bad and pulling back the engine.
    I don't think it would be a knock sensor. The ECU usually pulls timing on knock but doesn't limit revs. I think you'd be showing a code if there was excessive knocking.

    I think some old GMs used the tps more as a switch than a variable sensor, and relied on it to tell the ecu when to idle and when to come out of idle (or am I thinking of old Nissans... old somethings, anyway).

    It the circuit is open at idle and closed under open throttle, any interruption to the circuit could be trying to knock the thing back to idle, then closing the circuit and jumping back to life then back to idle.

    The other thing that doesn't make sense if they haven't changed it since running is the correct MAP sensor the ecu expects to see. If the ecu thinks 1v is an overboost situation and the map sends that signal at 2psi then you could have some issues. Unless the map was changed or a different checkbox was selected in the ecu setup than this shouldn't be it.

    -Justin
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  8. #8 Default  
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    Knock sensors should not limit RPM like that, I can't think of any application where they work in that manner.

    What about the balancer? If they start to go the pickups for the crank sensor can act funky.

    What does it have for an exhaust, could a cat be plugged if it still has one? I think those motors use some weird ass pancake pipe downtube off the manifold with a cat in it. The running rich part wouldn't help this condition for sure. Or are you absolutely positive it's a sensor/computer problem?

    Does it idle fine?
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  9. #9 Default  
    I don't know what constitutes a balancer going bad. I looked at the trigger rings and they all look solid and fine.

    I doubt it has a cat. Certainly doesn't smell like it has a cat. These aren't the type of guys to keep things like that when building a frankincar. Idles just fine. little bit of surging when you pull the MAF and it figures out "limp mode", but otherwise perfect idle.

    It shouldn't be the MAP. I didn't ohm it out, but we tried two different ones and same thing from both.
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  10. #10 Default  
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    Balancer is a good suggestion. I assume they'd be able to see an issue with that with a timing light.

    I'm assuming no cat.... because racecar.

    -Justin
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  11. #11 Default  
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    Some balancers have an inner hub, an outer weight and some rubber between the two. The rubber can deteriorate over time and allow the outer weighted ring to move around more than intended.



    -Justin
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  12. #12 Default  
    That wouldn't cause this. Balancer going bad on this engine just gives you knocking noise at the front of the engine. The trigger wheels are attached to the inner portion and won't float around with the outer section as the rubber breaks down.

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  13. #13 Default  
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    Most of the times I've pulled the crank pulley, there has been a separate trigger wheel. I would think you'd notice an issue with that particular damper before it got far enough out of line to move the trigger away from the sensor.

    -Justin
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  14. #14 Default  
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    My guess is the MAF wiring is internally shorted somewhere.
    "It was more passionate than just cuddling"
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  15. #15 Default  
    Quote Originally Posted by splatt View Post
    My guess is the MAF wiring is internally shorted somewhere.
    we started thinking along those lines and that's where I ran away. Wiring is bad enough, troubleshooting wiring is a nightmare.
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  16. #16 Default  
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    I would think that the MAF problem would show itself just off idle.
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  17. #17 Default  
    OK the only thing changed was the coil packs. If you have a lose (hi resistance connection on one of the wires) you will get crazy signals at the coil - Hence the bouncy tac. Trace all the coil pack wires. you may have a wonky wire

    also disconnect tac and check tac from scan tool. I know i can do this on a PC with my innovate scan tool. if you get the same results (should be the same) check the wires.

    if you pull the connectors you may see heat damage. Oh the other way to do this is run the motor and start touching the wires while you have your other hand on the body of the car. when you get a nice shock you found the wire. Oh make sure you pee first before doing this.
    Last edited by mthaynes; 05-18-2016 at 10:09 AM.
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