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Amedee, GMC Technician
Category: GMC
Satisfied Customers: 23935
Experience:  ASE Technician advanced level specialist
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GMC S15: Loss of Power - 93 GMC Sonoma – 4.3L (Z) – 186,000

Customer Question

Loss of Power - 93 GMC Sonoma – 4.3L (Z) – 186,000 Miles – 7/3/2012

SYMPTOMS: Intermittent problem. Low rough idle, poor acceleration, stalls, check engine light sometimes. The only code retrieved is 12 when using paper clip at diagnostic port. There is no odor (fuel, burning, etc.), vibration, mechanical noise, or apparent smoke present.

RECENT WORK: Last week: plugs (platinum – first time use), wires, distributor cap & rotor, ignition module, coolant temperature sensor, and catalytic converter. Three weeks ago: fuel filter and radiator.

DESCRIPTION: The engine did seem to develop a moderately fast idle for some time prior to the tune-up. Current problems developed shortly (less than 20 miles driven) after tune-up. Runs well 95% of the time but that percentage is decreasing the more it’s driven. The problems are more severe when climbing hills. If the throttle is advanced during these episodes, power loss (speed) is faster. There appears to be a “sweet spot” of the throttle position where speed is maintained well on level and downhill driving. When climbing hills, the “sweet spot” keeps me moving but speed decreases continuously. Of course if I reduce the throttle from the “sweet spot” the speed decreases.

The ignition module was replaced after the engine stopped running and the truck was towed back to the mechanic’s shop. There were two quick backfires before the engine shut down. No backfires since then.

My mechanic is, one might say, bewildered.
Submitted: 4 years ago.
Category: GMC
Expert:  Amedee replied 4 years ago.


Did you still need help?

When was the last time the engine had a complete tune up?

Does the engine ever stall on you at all when driving?

How does the engine idle under a heavy load?

Customer: replied 4 years ago.

Hello Amedee. Please define a complete tune-up.


If by driving, you mean in motion, no it hasn't stalled - but has at idle (parked, stop sign).


I just drove around the last two days with about 500 pounds of shingle waste before disposing and it generally handled the load fine.



Expert:  Amedee replied 4 years ago.

A full tune up would consist of plugs, wires cap and rotor.

Does your engine have a MAP sensor rather than a mass air flow sensor?

Have you checked the fuel pressure? Does it leak down after the engine is turned off?
Customer: replied 4 years ago.
Relist: Answer quality.
Amedee asked if iI had a complete tune-up. I asked him what a complete tune-up was. He responded: plugs, wires, rotor and cap.

These items, and more, were clearly listed as recent work completed in the body of my question. If he cannot comprehend what information was supplied in the question concerning the tune-up, then I am concerned about how he understands the rest of my question.

Need another mechanic. Thanks, Scott
Expert:  Amedee replied 4 years ago.

Sorry for the confusion. I now see that these items have been replaced already.

Have you checked the fuel pressure? Does it leak down after the engine is turned off?
Customer: replied 4 years ago.

Are you no longer concerned with whether it has a MAP or Mass Airflow sensor? How would I determine this?


I have not checked the fuel pressure and have no way to do so without going to a local shop.

Expert:  Amedee replied 4 years ago.
The mass air flow sensor will be in the air intake snorkel from the air filter to the throttle body.

If you don't have one of these, then you will have a map sensor. IT will look like this..

Click here
Customer: replied 4 years ago.

I could be wrong but my investigation indicates mass air flow sensors were used in 1996 and later models. A MAP is used for the 1993 4.3L (Vin Z) engine but I could not confidently locate it. In any event there is no sensor located in the air intake assembly other than the thermostat for the flexpipe leading up from the exhaust manifold.


I will have to see if I can get my mechanic to check the fuel pressure later this week.

Expert:  Amedee replied 4 years ago.
YEs.. thats correct! That sounds right. I would be willing to bet that this engine uses a MAP sensor.

A few things. First, we need to see if the fuel pressure is within specs when starting and idling the engine and under a load like when climbing hills. Both are very important. If you need the specs just let me know.

Also, we need to be sure that the fuel pressure does not leak down after the engine has been turned off. If so, then something in the fuel system is leaking fuel either back into the fuel tank or back into the engine not accounted for by the computer.

Another thing would be a bad oxygen sensor and air or vacuum leak under the hood.

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