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Strange Ignition/sensor Issue 1989 K2500


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#1 tatasta

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Posted 28 August 2009 - 10:37 PM

My son is now the proud owner of my old 89 Chevy 2500 4X4. It has a TH400 and a 350, which have both been rebuilt. It has ~ 80k on it and has always ran very strong with zero problems... Until recently.

A couple of weeks ago he called and told me his truck was running very poorly and that it had run fine the day before.

It started out looking like a fuel problem but no so sure now. I can get it to run and idle fair IF I unplug the MAP sensor. If I unplug the MAP and put it into gear, it will die.

BUT, if I leave the MAP unplugged (Vacuum line only) AND disconnect the TPS, it will continue to idle once I put it in gear.

I have replaced/inspected so far:
Fuel filter
TPS sensor
IAC
Distributor(complete including rotor and cap)
MAP sensor tested (I may have diagnosed incorrectly, however)
Plugs and wires
EGR is working

As noted, I can get it to run smoothly in park if I unhook the vacuum to the MAP but if I put it in gear it will bog down and die.

AND, if I unplug the MAP AND disconnect the TPS, I can put it in gear and it will continue to run. To the point I am pretty sure that I could drive it. But if I plug either of them back in, it will die.

I could use any input on this one.

Thanks in advance.

#2 bill190

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Posted 29 August 2009 - 10:16 AM

Idle is controlled by the IAC valve and the engine computer controls this...

And the TPS sensor, when it is fully closed, tells the engine computer to take over control of idle.

The TPS sensor is typically a "variable resistance" sensor which would allow very little electricity to "flow" at closed throttle and a lot of electricity to flow at wide open throttle.

There is a certain point/amount of high resistance/amount of very little electricity flowing - that the engine computer decides that the throttle is closed. (And then takes over control of the idle via the IAC valve.)

THEREFORE if the throttle is not completely closing (perhaps a bit of crud keeping it slightly open), or the TPS is not adjusted correctly (if there is an adjustment), or the TPS is malfunctioning and allowing more electricity through at closed throttle than it should, the engine computer would not know the throttle is closed! (And therefore not regulate idle.)

AND it sounds like when you are disconnecting the connection to the TPS, you are simulating a high resistance connection to the TPS, allowing very little electricity to flow, and causing the engine computer to then regulate the idle via the IAC!

Complete information on this would be in a GM Factory Service Manual set for your year/model vehicle (typically 4 different books to set). Order from helminc.com or a GM dealer.

Edited by bill190, 29 August 2009 - 10:17 AM.


#3 tatasta

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Posted 29 August 2009 - 10:38 AM

Idle is controlled by the IAC valve and the engine computer controls this...

And the TPS sensor, when it is fully closed, tells the engine computer to take over control of idle.

The TPS sensor is typically a "variable resistance" sensor which would allow very little electricity to "flow" at closed throttle and a lot of electricity to flow at wide open throttle.

There is a certain point/amount of high resistance/amount of very little electricity flowing - that the engine computer decides that the throttle is closed. (And then takes over control of the idle via the IAC valve.)

THEREFORE if the throttle is not completely closing (perhaps a bit of crud keeping it slightly open), or the TPS is not adjusted correctly (if there is an adjustment), or the TPS is malfunctioning and allowing more electricity through at closed throttle than it should, the engine computer would not know the throttle is closed! (And therefore not regulate idle.)

AND it sounds like when you are disconnecting the connection to the TPS, you are simulating a high resistance connection to the TPS, allowing very little electricity to flow, and causing the engine computer to then regulate the idle via the IAC!

Complete information on this would be in a GM Factory Service Manual set for your year/model vehicle (typically 4 different books to set). Order from helminc.com or a GM dealer.



Thank you for the great information. I will look over the throttle body again. It has been removed and "rebuilt" with a tbi kit. So it has been cleaned well.
One thing that I do notice is that when the TPS and the MAP sensor are connected, the idle surges very bad and I can here alot of suction sound in the throttle body. It sounds like a really loud vacuum leak but it is the TB itself. When I disconnect the vacuum line to the MAP, it will rev up some, idle back down a little, run smooth(er) and the sucking noise will go away. At this point if I put it in gear it will die. But then if I unplug the TPS, I then can put the truck in gear and it will continue to idle.

Why do you think that the truck responds to unplugging the MAP vacuum this way? It seems to be a problem between spark advance and choke/mixture.

I also unplugged the O2 sensor last night and it appears to be working properly.

Thank you for your time.

#4 redvett

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Posted 29 August 2009 - 08:46 PM

How about the codes? Might help.

Edited by redvett, 29 August 2009 - 08:54 PM.


#5 tatasta

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Posted 29 August 2009 - 08:59 PM

How about the codes? Might help.


Thanks for reading the thread. I have not seen any codes other than when I unplug thr MAP or TPS. No SES or codes when it initially happened.

#6 unioncoug

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Posted 02 September 2009 - 11:26 PM

I'm beginning to wonder about this problem with this style of truck yours is doing the same exact thing that mine is that is posted 95 gmc misfire, which is also tbi. except we've just replaced the map sensor because according to the chiltons book it should be reading with the ignition on from c and b .5 it mine is reading at a 3.49. Even when I start it it still it at 3.49 until I give it gas then it will drop in volts to .95 and up from there to full throtle. still at a loss what is wrong

#7 tatasta

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Posted 03 September 2009 - 07:55 PM

I'm beginning to wonder about this problem with this style of truck yours is doing the same exact thing that mine is that is posted 95 gmc misfire, which is also tbi. except we've just replaced the map sensor because according to the chiltons book it should be reading with the ignition on from c and b .5 it mine is reading at a 3.49. Even when I start it it still it at 3.49 until I give it gas then it will drop in volts to .95 and up from there to full throtle. still at a loss what is wrong



Well, I'm still messing with it. I have not been able to work on it much but will get on it in the morning. Got an ECT sensor code and picked up a new sensor yesterday. Need to reset timing as well.

I will post up with any updates.




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