100' in the valley and car started to rev high. What part is over heating?

[Some guy]

So i have this problem when it is 95 degree plus and i have been driving the car for a few hours. The motor will just rev to like 2500 to 3000 RPMS. I then have to let the car cool down for about an hour and not run the AC to make it home.

Yesterday i was driving all around. Stop start with the AC on. The motor stayed cool, about 180' from what i could tell. the AC worked as good as one can hope for in a 30 plus year old car. then driving down the street the motor started to rev high. it has happened before on the highway, on a very hot 95' day. So i pulled over to let it cool down for about 30 minutes. Drove home and let it sit.

I way not sure if it was the idle speed motor or the circuit board that controls it that was getting to hot. I tried to give the circuit board more air flow so the AC can cool it, but now i am thinking that is not it.

Has anyone had this problem?

[Bitsyncmaster]

So you mean the idle bumped up to 2500 RPM? If so, the first thing I would do is remove the thermistor wire on the idle ECU and make a jumper to replace the wires. If you don't have the pins, you could cut the wires and join them.

I suggest this because that 2500 RPM is what the idle ECU sets when that thermistor is not connected. You could use and ohm meter to find the broken connection but you really don't need that thermistor on the stock idle ECU.

[Some guy]

You know i wish i paid closer attention to the RPMs. I think you are right and they went to 2500. I am going to jumper it with a switch so that the next time it happens i can flip the switch and see if the RPMs drop to the proper range.

is the thermistor just for when the car is in cold weather and needs to warm up?

[Bitsyncmaster]

You know i wish i paid closer attention to the RPMs. I think you are right and they went to 2500. I am going to jumper it with a switch so that the next time it happens i can flip the switch and see if the RPMs drop to the proper range.

is the thermistor just for when the car is in cold weather and needs to warm up?

A switch sounds like a good idea. The thermistor with the stock ECU forces a "fast idle". Most people never run the car in weather cold enough to need that fast idle. I'm not sure if that thermistor controls the starting position of the idle motor in the stock ECU but that is what is does with my micro-ECU.

[David T]

A switch sounds like a good idea. The thermistor with the stock ECU forces a "fast idle". Most people never run the car in weather cold enough to need that fast idle. I'm not sure if that thermistor controls the starting position of the idle motor in the stock ECU but that is what is does with my micro-ECU.

When it idles high make sure the throttle arm is returning ALL the way back and hitting the idle micro.

[Elvis]

+1

[Some guy]

I was looking at the wires for the idle ECU that connect to the thermistor. In the wiring diagrams they are shown as Blue-gray and Blue-yellow. Mine are Black-gray and black yellow. There is also a resistance of about 1.5k Ohm. on it. This is when it is not running and "cold" about 80' garage.

Is that normal?

[Bitsyncmaster]

I was looking at the wires for the idle ECU that connect to the thermistor. In the wiring diagrams they are shown as Blue-gray and Blue-yellow. Mine are Black-gray and black yellow. There is also a resistance of about 1.5k Ohm. on it. This is when it is not running and "cold" about 80' garage.

Is that normal?

Just checked mine. Black/yellow and Black/slate read 7.86 Kohms at 83 deg F in my garage. You may have just read that 7 as a 1.....I had to look closely because I thought it was a 1 also.

My schematic shows the same color code I have.

So anyway it sounds like your thermistor is connected.

[Some guy]

ok perfect. i have the switch in place, once it happens again i am going to see if this take care of it.

[Jonathan]

Just checked mine. Black/yellow and Black/slate read 7.86 Kohms at 83 deg F in my garage. You may have just read that 7 as a 1.....I had to look closely because I thought it was a 1 also.

My schematic shows the same color code I have.

So anyway it sounds like your thermistor is connected.

I'm having my own thermistor related issue and thought I would ask in this thread if that's ok...

Dave, you mentioned the resistance coming in at 7.86 kOhm and 83 deg F. I tested mine and it was closer to 300 kOhm and in a garage around 73 deg F. I am pretty confident I read the meter correctly as it was scaling back and forth between 300 kOhm and 0.300 MOhm... but that is a lot more than what yours was.

I did the test with the car off and the battery disconnected (which that might be my problem, if it is, let me know) and I contacted the leads of the meter right at the harness side of the ECU connector while those two wires were not connected to the ECU on the other side of the connector. So with my meter reading between the black/yellow and the black/slate, going out into the car, apparently down to the thermistor and back, it reads 40x higher than your number.

Is this telling me anything?

[DMCMW Dave]

Dave, you mentioned the resistance coming in at 7.86 kOhm and 83 deg F. I tested mine and it was closer to 300 kOhm and in a garage around 73 deg F. I am pretty confident I read the meter correctly as it was scaling back and forth between 300 kOhm and 0.300 MOhm... but that is a lot more than what yours was.

Is this telling me anything?

That is way to high. Effectively open.

[Bitsyncmaster]

I'm having my own thermistor related issue and thought I would ask in this thread if that's ok...

Dave, you mentioned the resistance coming in at 7.86 kOhm and 83 deg F. I tested mine and it was closer to 300 kOhm and in a garage around 73 deg F. I am pretty confident I read the meter correctly as it was scaling back and forth between 300 kOhm and 0.300 MOhm... but that is a lot more than what yours was.

I did the test with the car off and the battery disconnected (which that might be my problem, if it is, let me know) and I contacted the leads of the meter right at the harness side of the ECU connector while those two wires were not connected to the ECU on the other side of the connector. So with my meter reading between the black/yellow and the black/slate, going out into the car, apparently down to the thermistor and back, it reads 40x higher than your number.

Is this telling me anything?

Yes your are measuring the thermistor correctly. No need to disconnect the battery but that will not change the reading. Your 300 K reading is way to high like Dave S said. Make sure your fingers are not on the meter probe ends when your reading high impedance values. Your fingers would lower the reading so you still have a problem.

[Jonathan]

Ok, thanks. Good to know the test was telling me something. The thermistor jumper can stay in for now then until I have a real reason to tear into the valley to connect it again properly.

[Some guy]

So it happened today, i was pulling out of my garage and about 2 minutes into my drive the car revved to 2500 RPMs. (it was around 70') I flipped the switch to force it to be a short and it stayed at 2500 rpms.

So i turned the car off, then back on. Stayed at 2500.

Did that one more time and it went to a normal idle. Drove home and parked the car.

I am going to make a permanent short over the thermister.

Is there any thing other then the thermister control that could cause the car to rev to exactly 2500.

I don't think it was the micro idle switch. it has been working good since i adjust the throttle linkage.

[TTait]

So you mean the idle bumped up to 2500 RPM? If so, the first thing I would do is remove the thermistor wire on the idle ECU and make a jumper to replace the wires. If you don't have the pins, you could cut the wires and join them.

I suggest this because that 2500 RPM is what the idle ECU sets when that thermistor is not connected. You could use and ohm meter to find the broken connection but you really don't need that thermistor on the stock idle ECU.

This exactly addresses the same problem I had - in my case there was a bad wire in the wiring harness, if I recall correctly its a green wire that hooks up to the thermister? In any case I swapped ECUs, Idle speed motors, butterfly valve, throttle spool and microswitch, installed a new thermister and did about everything possible before I went ahead and just replaced the entire wiring harness. It had some questionable repairs in it already and I wasn't really sure if it would help the problem or not - but that fixed it.

I wish I'd asked Dave. Let me tell you - if that fixes it you need to send Dave a nice bottle of something because I put a lot of time and money into that problem to fix mine.

Dave bailed me out though on a problem with my ac compressor spiking the system though, so I owe him a bottle too I guess! Dave - pm me an address.

[Some guy]

Thanks TTait

I forgot about the wiring harness, i tested my thermister and it was at 6K ohms. So i think it was good, but i was not even thinking bad wiring harness. I am not ready for that one yet, but it is good to keep in mind.

[Bitsyncmaster]

An open thermistor will bump the idle way up (my car went to 2500 RPM). You would want to short the two wires at the idle ECU to test it. That would eliminate the harness on that test. Only other thing that would bump it that high is a sticking throttle. I would not think the idle motor would stick that wide open. The idle motors have the most torque at the mid open position.

[dmcerik]

Thanks TTait

I forgot about the wiring harness, i tested my thermister and it was at 6K ohms. So i think it was good, but i was not even thinking bad wiring harness. I am not ready for that one yet, but it is good to keep in mind.

Some guy,

I'm a relatively new transplant to LA and live in NoHo/Burbank area. I've owned my Delores for 15 years now but don't have it with me. If you ever need another set of eyes on the Delorean while troubleshooting something please let me know. I might not have any good answers but I'd love to help and get some time around a Dleorean again.

[PJ Grady Inc.]

So it happened today, i was pulling out of my garage and about 2 minutes into my drive the car revved to 2500 RPMs. (it was around 70') I flipped the switch to force it to be a short and it stayed at 2500 rpms.

So i turned the car off, then back on. Stayed at 2500.

Did that one more time and it went to a normal idle. Drove home and parked the car.

I am going to make a permanent short over the thermister.

Is there any thing other then the thermister control that could cause the car to rev to exactly 2500.

I don't think it was the micro idle switch. it has been working good since i adjust the throttle linkage.
The most common cause of an intermittent sudden jump to a 2500rpm idle in my experience is a bad connection at the thermister itself. You will need to disconnect the Bosch 2 pin connecter under the intake manifold to check this. The bad news is for most people this is not an easy task epecially if you have big hands! That would be my next thing to check if I were you. It's not to uncommon for one of the barbed connections to start backing out of the plastic connector into the rubber boot that seals it. This causes precisely the symptoms you are describing. The good news is if you can get your hands in there it won't cost you anything to fix it. Except possibly some cramping or scrapes inflicted from the unfriendly confines of the dreaded "VOD"!!!!!!!!
Rob Grady

[Some guy]

For the time being, i am going to add a jumper. I live in LA so the coldest day i am going to be driving her is about 50'. Thank you guys for the help. I think you are dead on with it being some dirty connections. But my hands are to big to get in there and clean the connectors.

[Some guy]

So after it happened again on Monday. (The car reving to 2,200 and then almost dieing)

I cleaned the ICM connections, and the firewall connections.

These worked great
http://www.harborfreight.com/5-pc-airbrush-cleaning-brushes-68155.html

That didn't work.

It did it again.

So i pulled the ICB behind the driver seat. There was a cracked solder joint!!!!

I heated it up and i am keeping my fingers crossed.

It makes a lot of sense that it was something like that causing problems. It was on the main power lead coming into the board.

[PJ Grady Inc.]

That could be but usually ICU (ICB?) related failures are consistent so definately keep those fingers crossed. I'm still thinking a wire connection issue but I hope you're right.
Rob


So after it happened again on Monday. (The car reving to 2,200 and then almost dieing)

I cleaned the ICM connections, and the firewall connections.

These worked great
http://www.harborfreight.com/5-pc-airbrush-cleaning-brushes-68155.html

That didn't work.

It did it again.

So i pulled the ICB behind the driver seat. There was a cracked solder joint!!!!

I heated it up and i am keeping my fingers crossed.

It makes a lot of sense that it was something like that causing problems. It was on the main power lead coming into the board.