View Full Version : OT: Another S-10/S-15 Chevy/Gmc problem please -- finally got it fixed!!

Bill Pace
05-25-2010, 03:16 PM
Thought I'd respond to those who replied to my request for insight of the problem with this 89 S-15 2.5 4cyl, would start right up and run 3-5 sec and die - fuel shutting off.

Found oil pressure sender bad - needs oil press to allow pump to run - replaced it and still had problem, though the sender was bad!

After much googling, I had about reached the conclusion that the ECC (computer) was bad --- while googling, I ran across a Craig listing for an S-10 TBI for my truck. Called the guy way off in Greensboro, NC and in talking to him, he commented that he thought the ECC was bad also and that along with the TBI he had a computer also! Got together on price and sent the money for both the ECC and the TBI. This took about 2 wks to do but they arrived yest and I immediately plugged the ECC in ---- nope!, no change!

Was late, so waited til this morn to change the TBI, really not expecting it to make any difference.... but, it did, it cranked and ran normally:D.

So there are 4 parts that looks like could have caused the problem --The injector, though since it would put the 2 seconds of fuel in to allow it to start, seems doubtful that was it -- The throttle positioning sensor, the idle control valve and the pressure regulator assy. If I hadnt been so tired of messing with it, I would have changed them out one at a time and seen which it was, but .... maybe another day!

Thanks to all for the replies, I got some good hints/suggestions helping me do further research - (googling) - which eventually allowed me to get this thing back on the road.

05-25-2010, 03:37 PM
You got lucky on that one, Bill. 99% of the time, it's NOT the computer.

I remember when computers first showed up on cars, back in the early '80's.

We had stacks of computers sitting all over the place in the parts department. Every time a car came in with a problem, somebody was changing the computer. They would go back to the factory for testing and a rebuild....almost every one of them were found to be working as designed and the dealer was "back-charged" for the part and the warranty labor.
It was a frustrating time for anybody who did this kind of work.
Eventually, we learned how to perform proper diagnosis, and test various sensors and actuators. During my 45 years as a mechanic, I can't recall ever replacing more than one or two computers since. When I worked in an auto parts store, we would sell a reconditioned computer maybe once every two or three months, and then the odds of getting it returned were better than 50%. Of course, there are quite a few shops who employ "Trade School Wizards" who diagnose a problem by throwing parts at the car until it's fixed.

05-25-2010, 07:46 PM
One problem with testing components is that I have never been able to find values for the components, or even a compiled list of 'good' values for testing a car.

I was troubleshooting my old explorer a while ago, and after calls and letters to manufacturers, websites, and talking to various people failed to provide info, Typical response is drive up 'plug it in and read the codes'. Had too many 'techs' argue with me that the car is fine because the computer says it is. (if it's fine, why did it need to be towed here?)

I Finally resorted to taking my vom and notebook on a trip to different auto parts shops and checking the readings before I replaced anything.

Started taking readings from known good units and writing them all down, it's saved a hell of a lot of frustration, I now have known good values on a lot of different parts compiled on CD, all I need to do is find that CD.


05-25-2010, 08:55 PM
You're not the first person I've heard who couldn't find those "values" when testing a component.I looked after a whole fleet of Fords for 15 years before I retired, and never found a book or a chart with the "values" listed.

You're lucky in a way. Early Ford EEC systems required a "breakout box" to access live data from their computer systems. Damned "breakout boxes" were huge, and cost upwards of $300. But, they were handy, if you had one.

Ford was trying to keep the "shade tree" guys and the small "Mom & Pop" garages out of the computer diagnosis business, and force owners to bring their cars to an "Authorized Ford technician" instead. It didn't work. We (the "little guys" ) got clever and learned the systems, and soon the "breakout box" was a thing of the past.

An old boss of mine once told me that he could diagnose ANY computer problem in a car with only a DVOM...He couldn't.

An instructor at Ford told me to,"Just remember the reference voltage, (5 volts), and work from that."
5 volts is the sensor reference voltage, and oxygen sensors are 0-1 volt.
That's pretty much all you need to know, aside from the fact that everything the computer controls is brought back to the computer where it's grounded to operate. Ford, GM, Chrysler, the Europeans, and the Japanese all use very similar computers. In fact, most of the imports and Chrysler used either license built or stolen computers manufactured by GM, in the early '80's.

Bill Pace
05-25-2010, 10:24 PM
You got lucky on that one, Bill. 99% of the time, it's NOT the computer.

Salt, that is the case here, the computer was not the problem! -

Replacing the throttle body the guy sent, which had all the sensors still attached, was what fixed it... one of those sensors attached to the TBI was the prob.

J Tiers
05-25-2010, 10:51 PM
Had too many 'techs' argue with me that the car is fine because the computer says it is. (if it's fine, why did it need to be towed here?)


Ain't it the truth.......

If I hear "We can't find anything wrong, it's not throwing codes"......once more, it will be way too soon.

To be fair, there are so many potential feedback paths in a vehicle that it may be more useful to look for the component that would cause the OPPOSITE of the problem you actually see..... With a feedback loop, that can be more likely to spot the bad part...

The problem is that I don't know where to look for the "error signal" most of the time...... (2000 S10)

05-26-2010, 01:27 AM
My bad, I went back and read the post again. See what happens when you "expect" a problem? I should have known better, recently a friend of mine had a problem with his TBI Camaro....Pretty mush the same as you were having. Only his engine wouldn't stall, it would just idle rough, and "roll", until it warmed up. He was sure it was his IAC (idle air control). But, we soon discovered that it did the same thing with a new one. The scan showed the readings were correct, but the IAC wasn't "following" the ECM's commands.
A tug on the wiring harness found the problem. The wires in the plug on the IAC were broken and only an occasional signal was getting through. When the harness moved, the IAC went "open".
A new harness "pigtail" was bought, and soldered in....problem solved.

Sometimes the best diagnostic tools in the world won't replace a greasy fingered mechanic.

You probably won't get anybody to figure out the problem with your S-10, J.Tiers. At the trade schools (where all of these "technicians" are coming from) they don't teach basic diagnosis.
If it doesn't show a code, they don't know where to go from there. It took me ten years working on them every day before I really understood the WHOLE system, and how each sensor and actuator interplays. That's why it's so hard to diagnose something over the phone or online.

Here's a hint: The ECM won't set a code unless the "event" takes place for more than six minutes, or jeopardizes the emissions. That's why the "experts" never see most error signals.

J Tiers
05-26-2010, 09:43 AM
I don't have a problem now, I did have a bad TPS.

I also separately had a bad reading from the rear oxygen sensor..... THAT turned out to be a bad fuel filter...... the overpressure valve is in the filter, and it was releasing at the wrong pressure.

GM paid me to take that S10 off their hands........ the retail value of required warranty work was well over any profit they could have made......

But it has never had "unintended acceleration"....................;)