Idle Speed Thermistor Unplugged/Rough Cold Running

[Howard1398]

Hi everyone, my name is Howard and this is my first post. I'm currently fixing up my low mileage garage queen into a weekly driver.

Vin #1398 has been having cold running issues. Starts fine warm or cold and runs fine when warmed up, however when revving the car (whether under load or not) while cold the engine stumbles badly. It seemed to be a textbook case of a bad warmup regulator so I replaced it and nothing changed. Thoroughly examining behind the engine, I found the Bosch plug that goes to the idle speed thermistor unplugged (the back right side of the Y pipe underneath the intake manifold behind the 2 pin Lambda sensor). I can get the plug on the sensor easy enough but I cannot push it down into place with my two fingertips. I swapped the square shaped metal clip with the cold start relay plug clip thinking that would make it easier to push into place but no luck. Tried a variety of things too long to list here with no success.

I really don't want to pull the intake manifold. 1) Does anyone have any suggestions on plugging in the Idle Speed Thermistor Plug? 2) Is a rough/stumbling cold engine while revving consistent with an unplugged idle speed thermistor?

[Bitsyncmaster]

The idle speed thermistor would not affect the engine to run rough. Now if you mean the thermal throttle enrichment that would affect the lambda ECU which could cause your cold running problem.

[Howard1398]

The idle speed thermistor would not affect the engine to run rough. Now if you mean the thermal throttle enrichment that would affect the lambda ECU which could cause your cold running problem.

It's what is labeled as Idle Speed Thermister here:
http://www.specialtauto.com/delorean-parts/images/intake-manifold-under.jpg

The frequency valve isn't buzzing, no corrosion on the pins. Perhaps the Lambda system doesn't work without the idle speed thermistor plugged in? My dwell meter arrived today. I've never used one before. I'll see what I can find out.

[Howard1398]

Looking at this troubleshooting chart: http://dmctalk.org/attachment.php?attachmentid=8518&d=1329611095 , the unplugged idle speed thermistor appears to be suspect along with the idle speed throttle microswitch.

I jumped the RPM relay and the frequency valve does not buzz, only the fuel pump. Pushing on the full throttle switch does not make it buzz either.

I lightly clamped miniature vise grips to the idle speed thermistor plug before feeding the wire beneath the intake manifold, but there isn't enough room for both the vise grip and my hand in there at once. I'm hoping there's some sort of unorthodox way of plugging that plug back in.

[Rich]

Vin #1398 has been having cold running issues. Starts fine warm or cold and runs fine when warmed up, however when revving the car (whether under load or not) while cold the engine stumbles badly. It seemed to be a textbook case of a bad warmup regulator so I replaced it and nothing changed.
1) Does anyone have any suggestions on plugging in the Idle Speed Thermistor Plug? 2) Is a rough/stumbling cold engine while revving consistent with an unplugged idle speed thermistor?

Hi and welcome to D ownership and to this forum!

Answers in reverse order.
2. As Dave says, the idle speed thermistor is not what's causing the stumbles-when-revving-cold-only symptom you describe.
1. While I've never tried getting at it with the manifold on I do have a suggestion for trying to get the connector on. Get a long pair of locking forceps or a very long needle-nose pliers to grab the body of that plug and work it onto the idle speed thermistor.

The function of the thermistor is limited to acting as a signal to the idle speed control circuit to bump things up when the coolant in that part of the Y-pipe is <59degF. Only idle speed, not mixture. You can leave it unplugged if you will never be starting the car when temperature is <59degF, otherwise get it hooked up to avoid other problems, but not the one you are writing about.

Back to your stated problem. As you say, your sympton is sounds like classic case of the WUR being out of action. Likely it's a lean mixture during warmup. Check for vacuum getting to rear port of the WUR when cold (coolant <104degF), engine idling. A good WUR will do nothing in terms of cold mixture enrichment if it sees no cold vacuum at that port. This isn't the only thing that causes the cold-revving-stumble problem, just the main thing.

[Bitsyncmaster]

To get the FV to buzz you need to jumper the double yellow/red wires to the brown wire on the RPM relay connector. To run the engine jumpered, you need a jumper for that FV and the white/purple to brown wires. It will run with just the white/purple (fuel pump) but the lambda will not be powered.

[DMCMW Dave]

Another one nobody has mentioned - the ground wire at the back of the passenger side intake runner (tagged red, is about two inches long and there is an inline connector) is the ground reference for the O2 sensor. If that plug is disconnected or corroded, the ECU shuts down. That same ground line goes through the blue bulkhead connector.

[zimvsdib]

This isn't the only thing that causes the cold-revving-stumble problem, just the main thing.
What else would cause this?

I'm suffering from a very similar issue and its getting the best of me.
WUR is getting Vacuum until warmup. But my car still has rough warm up on cold starts.

Is my WUR operating correctly with these measurements?

just under 2 bar at cold start warm up
just over 4 bar after warm up.



example

https://www.youtube.com/watch?v=ZLuPItNYiP8

[Rich]

Is my WUR operating correctly with these measurements?

just under 2 bar at cold start warm up
just over 4 bar after warm up.

A more experienced forum member can perhaps answer directly. The trend is correct, though. Control pressure is lower at low temps.

What you might refer to in the meantime is D:02:01 in the Workshop Manual. It has all the details about testing the WUR/CPR. To start with you do the test with the vacuum hoses disconnected for the tests. Is that the way your video was shot?

[DMCMW Dave]

When cold, you should see a very noticeable "dip" in the reading as you speed up the engine. If this is not happening you don't get fuel enrichment and it will get the "falls on its face" effect. This will stop as soon as the car warms up (often less than a minute or two) so be sure to check for this cold.

If you don't get the "dip", it is most likely that the vacuum hoses are not connected properly (check vs the diagram on the engine cover). If that's all hooked up right, the ported switch under the intake may have failed although honestly that's pretty rare.

[Howard1398]

Thank you for all of your suggestions!

* The rear port of the warm up regulator has vacuum.

* Putting a jumper on the double yellow-red wires to the brown wire on the RPM relay connector is not making the frequency valve buzz. I then jumpered the white-purple to brown wires and the FV at the same time, but the engine would not start. After disconnecting the jumpers it took a lot of cranking and manually injecting fuel from the cold start valve using the specialtauto hot start relay with the cold start injector button to get the engine started.

* The ground wire on the back of the intake runner Dave mentioned is connected, but there is a very light layer of gray buildup on the flat side only. I scraped some of it off but nothing changed. I'll clean it properly tomorrow. Interestingly I plugged and unplugged this ground wire while the car was running with no affect.

* In zimvsdib's other videos his car behaves practically identical to what vin #1398 is doing, except my frequency valve isn't buzzing and the idle speed thermistor is unplugged. At cold idle my engine occasionally has some sort of high pitched hum that occurs. All the engine noises make it hard to determine exactly where that hum comes from. Revving makes it go away. I'm amazed my Delorean runs so well when warmed up with so much of the fuel system not functioning.

My attempts at plugging in the idle speed thermistor have all failed miserably, and I'm getting the scars to prove it!

[Rich]

The rear port of the warm up regulator has vacuum.

My attempts at plugging in the idle speed thermistor have all failed miserably, and I'm getting the scars to prove it!

Glad you are persistent.

Two things:
1. The idle speed thermistor, while nice to have connected for other reasons, has nothing to do with the cold-stumble problem.
2. I know you are saying there is vacuum at the rear port of the WUR under cold conditions. Just to rule it out as an issue can you please report the vacuum reading at the end of the hose during cold idle? And again after warmup? Maybe you already did report it. There is a follow-up question depending on the answer.

[zimvsdib]

rich my video was shot with the vacuum hoses connected to the WUR


Just to rule it out as an issue can you please report the vacuum reading at the end of the hose during cold idle? And again after warmup? Maybe you already did report it. There is a follow-up question depending on the answer.

I tested it again this morning with a vac gauge.
I plugged a vacuum gauge right in front of the WUR and i recoded 42 inches HG VAC then this slowly goes to zero after 3 minutes of warm up.

[Rich]

rich my video was shot with the vacuum hoses connected to the WUR
I tested it again this morning with a vac gauge.
I plugged a vacuum gauge right in front of the WUR and i recoded 42 inches HG VAC then this slowly goes to zero after 3 minutes of warm up.

Thanks for the data and the video conditions.

Full vacuum is ~30in.Hg, so I'm not sure how it can ever be 42in. anywhere, but that doesn't matter - unless your vac gauge is no good.

Let's agree that you have full manifold idle vacuum at the rear port of the WUR when cold, going to zero as the coolant in the Y-pipe warms up the vac switch to that hose. Sounds normal.

So now the follow-up question is whether the port at the rear of your WUR is open thru to the interior. You may have said you had it apart. In any case, try a short probe/wire into the port and see if you feel any blockage inside. Don't push too far. Reattach hose and try again.

After that it's on to the other port/hose/delay valve.

[Rich]

rich my video was shot with the vacuum hoses connected to the WUR
I tested it again this morning with a vac gauge.
I plugged a vacuum gauge right in front of the WUR and i recoded 42 inches HG VAC then this slowly goes to zero after 3 minutes of warm up.
Duplicate post, sorry.

[Howard1398]

Glad you are persistent.

Two things:
1. The idle speed thermistor, while nice to have connected for other reasons, has nothing to do with the cold-stumble problem.
2. I know you are saying there is vacuum at the rear port of the WUR under cold conditions. Just to rule it out as an issue can you please report the vacuum reading at the end of the hose during cold idle? And again after warmup? Maybe you already did report it. There is a follow-up question depending on the answer.

Thank you for all of your suggestions everyone!

From the end of the hose connecting to the rear warm up regulator port my gauge reads 15 in hg vacuum, after minute or so it crept up to just under 16 in hg vac. After another minute or so, before the engine warmed up, it dropped to zero. The moment it went to zero I heard the humming sound and I perceived the engine idle being slightly worse. I can't locate the sound with all the engine noise but its not located anywhere easy to reach.

With the engine off I pumped a mity vac brake bleeder in the vacuum system. I couldn't get it above 10-12 in hg vac, but it would lose negative pressure from 10 in hg vac to 5 in hg vac in two seconds. Not holding negative pressure would indicate a vacuum leak?

Is it safe to shoot compressed air into the vacuum system to find a leak?

Jumping the rpm relay with the engine off as Bitsyncmaster suggested didn't make the frequency valve buzz. There may be two separate problems here, otherwise it would have buzzed?

[hmcelraft]

Your problem is probably the vacuum delay valve in backwards. See the attached.

[Howard1398]

Your problem is probably the vacuum delay valve in backwards. See the attached.

It's on correctly, is there a way to test it?

[hmcelraft]

Put a vacuum gauge on the carb side of the valve. The leak-down should be 5-10 seconds (see specs). You should also check the Delay valve side of the pressure regulator to be sure the membrane in it is not ruptured and leaking vacuum. You can also apply a vacuum to the regulator - delay valve side - when the fuel pump is running to see if the pressure drops when vacuum is applied. Based on the video it would appear that the regulator is working - sort-of. The delay valve may be leaking down the vacuum to quickly. The delay time is important.

[Howard1398]

Put a vacuum gauge on the carb side of the valve. The leak-down should be 5-10 seconds (see specs). You should also check the Delay valve side of the pressure regulator to be sure the membrane in it is not ruptured and leaking vacuum. You can also apply a vacuum to the regulator - delay valve side - when the fuel pump is running to see if the pressure drops when vacuum is applied. Based on the video it would appear that the regulator is working - sort-of. The delay valve may be leaking down the vacuum to quickly. The delay time is important.

Vacuum test results with the car off in In hg vac:

Carb side of delay valve drops from 10 to 0 in 4.5 seconds. The distributor side of the delay valve drops from 10 to 5 in 60 seconds.

The front hose of the warm up regulator drops from 20 to 10 in 28 seconds. The rear port of the warm up regulator drops from 15 to 12 in 1 minute 40 seconds.

From the t adapter by the rear warm up regulator port to the thermal control valve no negative pressure (as there should be I think).

From the solenoid valve to the thermal control valve pressure dropped from 15 to 10 in 30 seconds. Other solenoid valve hose to the spark advance holds pressure. The reverse, spark advance hose to the solenoid holds no pressure.

After warm up (when everything seems to be normal):

Vacuum at idle from the solenoid valve to the thermal control valve is 16 in hg, and there is no vacuum from the spark advance to the solenoid valve. When revving vacuum from the spark advance is 2 in hg.

[hmcelraft]

Are you sure the hoses are routed correctly? Vacuum to the CPR only when cold (<40C)and vacuum to Distributor Vacuum advance only when warm (>40C). See page 88 of tech manual.

[PJ Grady Inc.]

After all this back and forth on vacuum hose routeing I'm going to assume you've conirmed the routing is correct and functioning at the proper temps and move on to the problems you know you have!

The disconnected idle switch is something I've encountered many times and I've usually been able to connect it with my somewhat long and skinny fingers! If "you have short fat pudgy fingers" (as John Whochamacallhim said in a movie who's name I can't quite remember) that could be the problem! Since I am presently unable or willing to part with my hand for you to try maybe you can find someone with the proper hand shape to attempt this in my stead. It's an important engine system and IMHO could be contributing to the cold stumble through lack of sufficient air flow when cold.

The inoperative Lambda curcuit is also extremely important for driveability both hot and cold. You should concenrate on finding the cause for which there are numerous possibilities. Let me give you some examples gleaned from a roadtrip to Ohio to help Ken Koncelik clear out an excess number of Deloreans loitering about his warehouse with undiagnosed drivability issues. On three of the first four cars I worked on the Lamdba curcuit's were inoperative. I found three different causes between the the four cars with one car having two reasons not to function. They are as follows in order of diagnostic difficulty.

1) Failed point contact in the RPM relay. (Common)
2) Failed Lamda ECU. (Uncommon)
3) Failed Lamda ECU caused by a high resistance ground issue. (Off the charts in diagnostic terms)

The poor ground would allow the circuit to operate with the new ECU but only when grounding the O2 senor WITH THE ENGINE OFF! There was a high resistence in the ground between the ECU and the plug in manifold ground that Dave referred to earlier. The RF signals from the ignition circuit would overcome the poor grounding but only with a running engine. The correct repair would be a new engine harness but I cheated and took the less expensive route. I'd tell you the secret but then I'd have to kill you;) Anyway that particular car absorbed the better part of a day to figure out and your problem is probably simpler. At least I hope so for your sake! Another common cause is a bad or misinstalled Lamda relay. Check those things and then you may have to play with the CO adjustment if it was tweaked to mask the cold running problems.

In short fix what you know is broken first and your issues will be solved.