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After ECM reset great power-performance for about 30-minutes

1994 3.1L V6 (180hp) Chevy Corsica…

After I reset the ECM computer, the car performs like a former police-car, lots of power-performance, but then with the passage of time, about 30minutes it 'learns" or goes back to under-powered state with weaker performance, … I have maintained well maintained car getting up to 38MPG with 92-97 octane gasoline driving economicaly, and the only clue I have to loss-of-true-power-performance is that the temp-sensor (with-pigtail) responsible for dashboard engine-temp display is likely responsible for showing on dashboard display 1/8 inch below minimum mark (on display) when engine cold and at most 1/4 way on then entire display range (scale) under maximum-engine-heat-conditions…

v since OHM tested the temp sensors, there are four, one air-intake-temp-sensor I cleaned and it read a normal ohm reading, I did not remove the other sensors mounted on engine, but one sensor read nearly 10K OHMs when engine was cold at 45F and the other sensor read 5.6K OHMs at same 45F temp, the third sensor that controls electric cooling fan I am not worried about because that fan turns on properly when engine is very warm… There is coolant in the system, so not a low-coolant or air-pockets… Why can’t it run like a rocket all the time…

Could I have a problem with one temp-sensor sending a 10K OHM reading and a different temp-sensor sending a 5.6K OHM reading for the same 45F temperature, does the ECM use both or an average of both…

Thank You…

There’s no way to know without checking it against a calibrated thermocouple/meter.

Without more information as to how you came to this diagnosis, I’m not confident in thinking that the sensor is the cause of your problems. It’s possible that the reason the temp needle on the dash is reading below the minimum is that the system is low on coolant and the sensor is in an air pocket. This will cause a low reading. What have you evaluated in the cooling system?

temperature sensors can be checked for accuracy using an ohmmeter . . . if you have the proper chart, which shows the correlation between temperature and resistance

That could be found in the factory service manual or a Chilton/Haynes manual

I’m not sure I follow but if you are saying the temperature gauge in the car is reading too cold, it would likely be more of a thermostat problem than a sensor problem. There are two temp sensors though. One just tells the gauge what the water temp is. The “engine temp sensor” though tells the computer what the engine temperature is though so the computer can set the correct fuel air mixture for the conditions. This or other sensor problems, like MAF, air, O2, etc. could be providing bad info to the computer. The only way to tell though is with computer diagnostics to see exactly what the sensors are reading compared to what they should be reading.

Why use that ?

True. And perhaps that supports the idea that the coolant is low… leaving both sensors in air.

I use higher octane gas to get better gas mileage performance…

Iv since OHM tested the temp sensors, there are four, one air-intake-temp-sensor I cleaned and it read a normal ohm reading, I did not remove the other sensors mounted on engine, but one sensor read nearly 10K OHMs when engine was cold at 45F and the other sensor read 5.6K OHMs at same 45F temp, the third sensor that controls electric cooling fan I am not worried about because that fan turns on properly when engine is very warm… There is coolant in the system, so not a low-coolant or air-pockets… Maybe I should keep it in D when in town and under 50MPH and only put transmission into OD when highway or greater than 50MPH driving… but still mysterious that it runs like a rocket for first 1/3 hour and turns into a whimp-normal-car after it learns each time after the ECM is reset… Why can’t it run like a rocket all the time…

Even Consumer Reports states that is not true. The difference a vehicle designed for regular 87 and using a higher octane the miles per gallon increase almost can’t be measured. Low tire pressure will have a greater effect on MPG.

Also your vehicle will activate OverDrive in D position and you have to turn it off which will use more fuel.

I suspect a fuel deliver problem rather than a failing sensor. The problem with a car that age is that there is no fault code criteria for low engine temp, cylinder misfire, or sensors reading out of range unless it’s a complete sensor failure (usually).

I think you’re going to need to find someone with a scan tool capable of giving you live data, specifically fuel control numbers like block learn and integrator counts. A lot of shops don’t even have that capability anymore.

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Sorry, I have not been informed before today, that using higher octane gasoline is not better for my gas mileage, I’v only relied thus far on my own experience on five motorcycles from 16-23y-old (GS1150e=fastest) and various automibiles I’v owned thus far, with one exception, a 454ci chevy TBI motorhome did not perform better with higher octane gasoline… also nearly all european countries have as a minimum of 95 octane gas at the pump, meaning not possible to by any lower octane gas even if you wanted to and the best are 98-103 octane benzine at the pump … so go figure…

I think if the fuel delivery system was bad then it would cause a fault all the time, here we have great powerful first 30 minutes after ECM reset, then go quiet the rest of the time, for ever… Can I have a problem of two engine temp sensors sending wildly different OHM readings for same engine temperature, then the ECM backs off and plays it safe, very conservative, until someone discovers the problem and replaces one of the temp sensors that are wtong for this car… ???

-tibor

Are you by chance Hungarian . . . ?

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On the older cars I have experienced failing oxygen sensors that send a false rich signal to the computer, the computer leans the mixture to the point that the engine has poor power output. Try driving with the oxygen sensor unplugged, the computer will operate using base line values and won’t adjust the fuel trim.

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Igen, persze, illetve termeszetesen, (Szekely-Magyar) 1982-ben erkeztem Amerikaba es az ota tanulom az angol nyelvet, de szerintem mar bele jiottem jo alaposan, es meg en reklamalok hogy a Magyar-orszagi Magyarok tul sok idegen szavakat kevernek bele a minden-napi beszedjukbe…

-tibor

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@db4690, the OP’s name is traditional Hungarian. I am wondering if he is not in the US as he also calls gasoline benzine - very Euro name for gas. This may be the problem in getting a decent diagnosis, local mechanics may not be familiar or have the right tools.
I think @asemaster is on the right track.

I guess so . . . I didn’t understand a word

I was asking because a former colleague of mine was from Hungary, and he had your name :smil

I agree that @asemaser has given you some good advice

I had a O2 sensor that would richen the mixture to the point where I was getting about 12 mpg. I didn’t notice any performance problems but when I put a new sensor in for $30 and took it on the highway, it took about 5 miles and I was getting 27 mpg. Might have been why the guy before me sold it.

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That may be the opposite of what the OP is experiencing, you are going to confuse the issue with your personal anecdotes.

OK… all great advices! A big thank you for everyone!

I’m going to check the OHM data on what they really should be and what the actual sensors are sending, and I am going to try to make sure they are all sending the right data to ECM, right now I can only think of this to be causing confusion to ECM and then it backs off and does nothing performance oriented, after about 20-30 minutes of learning, the O2 sensor is somewhat new, only a few years old, EGR-valve taken apart and carbonized and put back together, MAP sensor could be next possible after verifying TEMP-sensors singing the same tune or about the same OHM reading for one temperature… because two of three is reading around 5.6k OHMs at 45-50f temperature and the third said around 10k OHMs …

-tibor