Let's talk about high idle and why it wont come down

[Justin51982]

So my car is running (a little rich) but idle, whether cold or hot stays around 1500rpm (this is after I got it down from 2200rpm but messing with the 2 idle screws that hit the micro switch.

I talked to Dave at MW a little bit about this, but I could tell he was busy so I didn't want to press him.

Any thoughts or ideas?

Car has new plugs, cap and rotor and plug wires. Car starts right up if cold or hot and I've gone through the vacuum system and unless I really just overlooked something, I believe that the entire system is in tact.

Any ideas on where to start and/or how to test certain things?

[Jonathan]

Justin,

My experience with a high idle right at 1,500 RPM is that it was related to the connections on the back of the ECU behind the driver's seat. And for me, was specifically the thermistor connections.

I looked back at my notes and there was mention of a way to test this from talking with Dave, but I don't see details on what that test was.

If it isn't an idle that is sticking and snapping the gas or pressing in the throttle spool to hit the microswitch doesn't bring it down, it could be your thermister connection isn't good.

The thermistor is accessible in the valley (http://store.delorean.com/p-6749-the...dle-speed.aspx) but not so much without tearing into the top of the engine. I wasn't prepared to do this quite yet, so I rigged up a jumper that bypassed it at the back of the ECU.

If you look at the ECU behind the driver's seat (http://store.delorean.com/p-6720-idle-speed-ecu.aspx), the connection on the passenger side with only four wires going to it is the one you want. I believe the thermistor comes and goes on the pins marked 9 and 11. What I did was put two short lengths of wire on the other two points to make a bridge for those two and keep their connections good, and then a third short length of wire to jumper on those thermistor pins right at the back of the ECU. What you end up doing is tricking the system into thinking it has an okay signal from the thermistor which got unplugged. There is some sort of implication based on running the engine in cold weather, but I wasn't too concerned with that.

Maybe someone here will chime in with what the quick test is to help you, but that MacGuyver jumper set-up is what I still have on mine and has been there for over a year. I think I recently realized the connection coming from the blue wire out the engine bay grommet is this same thermistor connection, so who knows, maybe it was only that part that was accessible this whole time that was the bad one.

[Bitsyncmaster]

I've found that an open idle speed thermostat runs the idle at 2500 RPM. Also found if your getting vacuum advance at idle the RPM is 1500 RPM. But those RPM values can be affected by a lot of throttle adjustments.

First thing to try is pull the vacuum hose off the distributor advance and see it that helps.

To test the thermistor, pull that plug with the 4 wires off the idle ECU and use an ohm meter to measure the resistance on the pins with black/yellow to the black/slate wires. If the engine is hot you will read about 1 Kohm. If the engine is cold you will read about 50 Kohms. If you read infinity, you have a broken wire or disconnected thermostat.

So I assume you have the lower "curb idle" screw backed all the way out. And your upper screw (on the micro-switch) is backed out just enough to just actuate the switch.

[David T]

This can happen when the quadrant link ends get worn and develop a lot of play (looseness). You may have adjusted the set screws but if the throttle arm is not being returned all the way back to idle the motor will still not idle even though you have adjusted the set screws. Check for play and if you see it lengthen the quadrant link so the throttle arm is pushed all the way back to idle.

[Jonathan]

To test the thermistor, pull that plug with the 4 wires off the idle ECU and use an ohm meter to measure the resistance on the pins with black/yellow to the black/slate wires. If the engine is hot you will read about 1 Kohm. If the engine is cold you will read about 50 Kohms. If you read infinity, you have a broken wire or disconnected thermostat.

Good on you, Dave. I think you just helped me figure out what might be the problem area on my car.

I went out quickly to the garage and tested the thermistor end of the wiring at the connector. I get open circuit/infinity on that test. I went back into the engine bay though and when I did the test on the thermistor end of the connector, it gave me just under 1 kOhm (about 0.6). My car is off and engine is cold, but it's very warm here now outside. The bulkhead connectors schematic says that circuit goes at 52 deg F, so I was thinking that even with a cold engine in my hot garage, it would read closer to the 1 kOhm. Sound about right?

Anyway, I tested the connector that I had disconnected, and a couple times it read open loop/infinity when I was pressing the leads on the two tabs, so I think there could be something flaky right at the connector on the thermistor wire side. I'll poke around further and see what I come up with.

But back to helping the OP...

[Bitsyncmaster]

The bulkhead diagram shows 14.6 Kohms at 52 degrees F for the "Idle speed regulator switch". I wish they did not call it a switch, it never should be open (infinity) or closed (0 ohms). So testing that resistance on the bulkhead, you will want to pull that white bulkhead connector and measure the resistance from the black/slate (#46) wire to the black/yellow (#50) wire. You should get something around 10 Kohms (depends on your air engine temp).

If that resistance does not match what your read at the idle ECU connector then you have found a problem with the wiring or the bulkhead pins were not mated very well.

Now if you read infinity at that bulkhead connector then most likely your thermistor is unplugged in the "valley of death".

Someday I will need to measure the thermistor related to air temp and record that table for us to use. I looked over my ECU software and did see a comment of 25 Kohms at 30 deg F.

[sdg3205]

I also came across a car once where a PO had backed out the brass plugs/screws on top of the butterfly assembly, probably thinking they were for adjusting idle. I could not for the life of me figure out why the idle was so high, then i brushed one of the plugs and it moved. Bam, idle fixed. Quick and easy to check.

[Justin51982]

I did the cooling system flush last night and checked a couple things but not everything. I checked the brass screws and the idle adjustment screws, even going so far as to back them all the way out. I also feel the car is running too rich, went to lean out the mixture and it didn't like that, kept trying to sputter and die, so I just put it back.

During the flush, I pulled the engine drain plugs, both were clogged and after poking around I was able to dislodge the sediment , finished back-flushing and refilled and bled the system.

My point to this is that when I started the car up, and for about 20-30 min the idle was sticking around ~750-800 (immediately after the flush and refill)..After warming up last night the idle went back up to ~2000rpm. Went out there this morning, decided I would take it to the tag office to get a tag for it and from start up on, idle stayed at the ~2000 mark.

I will try pulling vacuum off the dizzy tonight and testing the ECU tonight.

[Bitsyncmaster]

Does the idle motor buzz when your engine is running?

[Justin51982]

Not that I've noticed, but between the high idle and the fact that it has headers, I could have easily not noticed.....I'll check that this evening as well...

Dave, you don't want to come down to San Antonio do you?