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Cold nights cause a temperature-dependent max achievable speed

This had been a problem for a long while, outliving all repairs I have done on the car. Lately I have to drive on a daily basis, and this winter the problem became a royal pain.

The colder the night was, the lower will be the maximum speed I can get to. At that max threshold speed the car would buck, jerk as if ignition is not happening, and can even stall, if forced.

The oddity is that warming up the engine on idle has little effect, compared to driving under max achievable speed. After about 5 minutes of driving, that maximum speed will start increasing, after 5 more minutes it will gain another +10mph, and in about 30 minutes I can drive fine at any speed until next cold night.

There are no error codes, starting, idling, stopping, accelerating, going at 80mph are all fine, there's no jerking or stalling other than in the situation described. At warmer weather I won't notice the problem, or that max speed would be around 70mph, where it doesn't bother me so much.

This issue survived:

  • transmission rebuild
  • engine change
  • three MAF sensors change
  • spark system change (plugs & wires)
  • ICM change (ignition control module)
  • O2, camshaft, crankshaft sensors change
  • new catalytic
  • new injectors
  • fuel filters change

Throughout the years it's as solid as my appetite. And it's as disturbing. Imagine how you wake up in the middle of winter, you need to drive to work, and you aren't able to drive above 20mph for good 5 minutes. Now imagine that you're a police officer, sitting in your vehicle, enjoying your morning doughnut, and then a rusty old car crawles right in front of you at a treacherously low speed, and then you see it again and again, as I make several circles around the area trying to get the car to the speed where I could pull on the highway. This picture can make even the most breakfast-minded detective concerned.

I'm looking for any pointers, anything I could try to diagnose what the hell is so profoundly wrong with my otherwise great beast.

It's a 1988 GM with 75K engine, no distributor (coil packs), injection, V6.

Reply to
Office Drone
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About the only thing not yet changed out is the fuel pump. Check the pressure at idle when cold.

Reply to
« Paul »

If it is a 3.8L, You need a new "reluctor" on the camshaft gear. This is a small magnet that is held by a plastic clip that gives the cam sensor its signal. Every time it rotates around past the cam sensor, the sensor changes voltage and thus makes the "pulse". I have found almost identical symptoms in several cars like yours that need this repair. The magnet over time deteriorates and either falls out of the plastic clip or just turns to mush and the sensor no longer picks it up.

Reply to
noname

I heard the same suggestion from a dealer a year ago.

But why a weak or deteriorated magnet would be so dependent upon the air temperature?

Reply to
Office Drone

Certain magnets have been known to change their properties when heated or cooled. It's quite possible that the temperature changes in your car are having a similar, and affecting the behavior of the car as a result.

Reply to
Isaiah Beard

Sounds like the throttle body is icing up....

Reply to
Sportster4Eva

Engine pressure is good, but since my oil pump had been noisy for quite some time, and I had a cheap one handy, replaced the pump, fuel filter, checked for restrictions on filter & fuel line. Everything should be fine with fuel delivery now.

Yet the cold temperature/max speed problem persists.

Paul wrote:

Reply to
Office Drone

Doesn't this vehicle have a pump in tank? What is the rail pressure when cold? Does the engine rev high in first and second?

Reply to
« Paul »

Reply to
SgtSilicon

I began to get this same problem with my 1991 3800 series engine, but it began to be very hard starting as well. But today it will not catch at all, I had a spare coil pack and it made no difference, not sure if ICM was ever replaced, nor the crank sensor. So where is the crank sensor located?

Reply to
Danil

Have you replaced the coil pack? Mine were bad in late summer and engine balked and hesitated when under load.

Reply to
Danil

Yes, we put a coil pack from a different car in the garage. Changed plugs&wires. But mine ran the same exact way next morning.

It was raining today, and I couldn't get even above 15mph, the car stalled 5 times before I could make it to the end of the block.

Last machanic checked:

  • delivery system
  • idle speed
  • EFE system including thermo air cleaner
  • EGR system
  • individual cylinder condition
  • primary & secondary ignition circuits
  • dwell, base timing & timing advance

And it all checked out.

The top speed I can get to has an almost linear dependency upon air temperature. There are no error codes either.

After something magical warms up (or burns up) inside the car, it runs without any problem for the rest of the day.

What else could it be?

Reply to
Office Drone

That would make sense. How can I diagnose this on my own (to make sure), and how does it get usually fixed?

Reply to
Office Drone

For what I'm reading, in such case after cam- or crank- shaft sensor can't read the magnet for 5 seconds, they raise an error code. My car doesn't have relative error codes. However, I was getting CamShaft errors when my ignition module was bad. After I replaced it, everything is clear.

Also, wouldn't a different magnet be inherited with the new engine?

Reply to
Office Drone

Hi...

Here I go again... not a mechanic, just an old retired electrical guy who loves tinkering with cars, so pay little attention, please. You've been warned :)

Maybe a little advantage in this case... a simpler perspective...

You didn't mention the Intake Air Temperature sensor, nor the Coolant Temperature sensor... they should (I think) set codes, but might it be worth changing them on a whim ? Perhaps you didn't get new ones with the engine change.

I'm particularly thinking this way because I live in real cold Canada... and if I go out and start my car, let it idle 15 minutes while I have yet one more coffee, the car hardly warms at all... air temperature out the vents stays no better than lukewarm. Yet driving it loaded for a couple of blocks has really nice hot air available.

Just a thought...

Take care.

Ken

Reply to
Ken Weitzel

Hi yet again...

Sorry, didn't notice the vehicle age.

Shouldn't have said air intake temperature sensor, but rather manifold air temperature sensor for an '88

My apologies.

Ken

Reply to
Ken Weitzel

Reply to
Silver Surfer

Not really. I would say, the spread of temperatures causing the problem would range between 40F(and below) up to 70F. So the warmer it was, the higher would be the speed at which the car would stuck up.

Reply to
Office Drone

OK. That rules out something freezing up then. Can we assume that your machine is not garaged?

Assuming that the vehicle is close to a source of electricity have you thought about positioning a 100W light bulb at various places in the engine compartment overnight to keep specific areas warm. Might be able to narrow down the list of suspects that way.

Reply to
Silver Surfer

Can't get your problem off my mind.

You mentioned that the engine was replaced in your machine. Which components from the original engine were reused on the replacement engine?

Reply to
Silver Surfer

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