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Old 04-05-2011, 07:12 PM   #1
boo-koo
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2001 SC1
Default Engine revs increase when going downhill

My daughter has a '99 SL2 w/ 130K miles, in overall good shape. Today she informed me that while driving on a hilly road near our house that the engine suddenly revved up to between 3-4000 RPM, but only while she was going downhill.

So she and dad went for a ride.

The tranny fluid looks fine and is at the proper level. The car drives and shifts OK and it pulls itself up a hill normally. But, sure enough, as we were descending the last hill before we got to our road (coasting with light braking), the tach shot up to about 3200 RPM and didn't return to normal until we hit level ground -- about the same time we had to stop at an intersection.

This is a new one on me. Any ideas?

Thanks.

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Old 04-05-2011, 07:38 PM   #2
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Default Re: Engine revs increase when going downhill

When parked what is the idle in park or neutral?

You day coasting. Was the transmission in gear?

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Old 04-05-2011, 09:24 PM   #3
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Default Re: Engine revs increase when going downhill

That sounds completely normal to me. The software in the pcm will sometimes shift to a lower gear when going down a hill, it allows for engine braking to slow the car down and not just the braking system.

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Old 04-05-2011, 09:44 PM   #4
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Default Re: Engine revs increase when going downhill

That is what I suspect also but maybe not. That is why I was asking those questions.

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Old 04-06-2011, 03:56 AM   #5
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Default Re: Engine revs increase when going downhill

Coasting = in gear, foot off gas.

Idle revs are around 1000 (a guesstimate, I can find out exactly later today). Doesn't seem overly fast.

No, this was not normal downshifting / engine braking. The engine (DOHC) revved beyond what a downshift would have caused. We're talking about 3500 RPM at ~30 MPH here. Further, there was no sensation of engine braking or being held back by lower gearing.

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Old 04-06-2011, 08:33 AM   #6
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Default Re: Engine revs increase when going downhill

Your idle should be ~850 in park/neutral and ~750 in drive drive. If it is ~1000 RPM then there is a problem.

So, has the ECTS been replaced? Where does the temperature gauge read when the engine is warmed up and you are driving down the road at 35-45mph?

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Old 04-06-2011, 06:47 PM   #7
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Default Re: Engine revs increase when going downhill

OK, engine idles at a little under 900. Temps are normal, a bit above the 1/4 way mark on the gage.

My experience with the ECTS (which has not been replaced since we've owned the car, about 3 years) is that temp readings and revs are erratic when the ECTS acts up. In other words, I've never seen it produce an event like this where the engine only revs up when going down a hill.

When that occurred, BTW, the temp gage did not fluctuate. We had descended two other hills prior to the last one and the car did not misbehave on those.

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Old 04-06-2011, 08:12 PM   #8
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Default Re: Engine revs increase when going downhill

Actually, a failed ECTS usually produces NO detectable symptoms that are visually related to the temperature indication. I would replace it and then if you do not reach 3/8 on the temp gauge replace the thermostat. And in drive you should have a 700-800 RPM idle and 800-900 in neutral. Should have both conditions. When coasting the engine will hold 1200 to 1400 RPM and if the trans down shifts it will increase. If the ECTS has failed and is providing a cold temp signal then the RPM will be higher. The ECTS is a $10.00 part and 20 minute replacement. Testing it for accuracy will take longer and require equipment not usually found in the home shop. And if it is plastic it is junk anyway.

There is a problem here as the coasting RPM is high but what is not instantly obvious.

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Old 04-07-2011, 01:36 AM   #9
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Default Re: Engine revs increase when going downhill

I don't know about 3500 rpm, but my '95 SL2 will hit at least 3000 rpm for engine braking, going down hills. There's not going to be a lot of engine braking, regardless--it's a small engine with little mass.

If the temp reads right, consistently, the ECTS and thermostat would have to both be bad at the same time, in inversely corresponding amounts. (ie. if the ECTS read 20 degrees too high, the thermostat would have to open 20 degrees too low to make the gauge read right.) This is unlikely. (This is un-true on OBD1 ('95 and earlier) cars, which had two sensors. Also, it's pretty easy to test--all you need is a multimeter. Check it for resistance cold and at a few points while warming up. If it reads right cold and at operating temperature (wait for the radiator hose to get hot signaling the thermostat opening), chances are it's fine. You can find the values in a chart someone uploaded to the pics section (or in any chilton's manual). Obviously, if it's the old resin type, replace it regardless.

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Old 04-07-2011, 07:57 AM   #10
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Default Re: Engine revs increase when going downhill

Quote:
If the temp reads right, consistently, the ECTS and thermostat would have to both be bad at the same time
Not really, the ECTS does not fail by having a linear offset to the correct resistance across the ambient to hot operating range. By you logic a person could spot a failed ECTS from gauge readings and this is not true. Only a gross failure is obvious on the gauge.

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