Therm Vac Switch 102405, How Often Do These Go Bad?

[kings1527]

12270

Hi all. How often do these need to be replaced? DaveT, DaveM, and DaveMidwest, I'm sure you guys have a good idea. I'd appreciate anyone's opinion. I have the intake manifold off right now and in the middle of detailing/POR15 the valley along with a slew of other things so there would be no better time than now to replace it if it's a common failure point. But at $40, I'd rather get some opinions first. The only thing that was funky at the time I broke my engine down was I had a high idle (about 1000) once warmed up. Could this have been the issue, aside from most likely a vacuum leak?

Thanks.

Alex

[jawn101]

12270

Hi all. How often do these need to be replaced? DaveT, DaveM, and DaveMidwest, I'm sure you guys have a good idea. I'd appreciate anyone's opinion. I have the intake manifold off right now and in the middle of detailing/POR15 the valley along with a slew of other things so there would be no better time than now to replace it if it's a common failure point. But at $40, I'd rather get some opinions first. The only thing that was funky at the time I broke my engine down was I had a high idle (about 1000) once warmed up. Could this have been the issue, aside from most likely a vacuum leak?

Thanks.

Alex

High idle at 1000... was the electrical connector to the vacuum solenoid attached properly?

[DeLorean03]

Here's a good way to put it. Back in 2004, I was doing a VOD restore, and we decided to not replace the idle speed thermister. I had never heard of one going bad - figured at $35, not needed. Guess what failed and had my RPMs at 2500 until we learned to bypass the idle speed thermister by rewiring the idle ECU....

Also, on another note - DMCMidwest is installing a new transmission in my car right now - at least the "head portion" where all the hydraulics are located. While it's out, the trailing arm bolts have never been serviced in 30 years. Now, what's the failure rate of those bolts...? Pretty low. Than again, my car could look something like this if those 30 year old bolts break:

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My opinion: you've torn down the engine to the VOD - pop $40 and replace it. You will know it's done, and you won't have to come back underneath the manifold. At least, that's what I'd do for the peace of mind it'd give me :).

[kings1527]

High idle at 1000... was the electrical connector to the vacuum solenoid attached properly?

Thanks Jon. As far as I remember when I checked things in the engine compartment when I was troubleshooting, and as far as I can remember when I was removing the intake manifold, yes. I remember seeing one vacuum line (can't remember which) that was cracked at the hub and for all I know, that could've been the source. I'm going to order Hervey's silicone vacuum replacement line set for $50 and be done with that forever. He gives a lifetime warranty on his vacuum lines.


Here's a good way to put it. Back in 2004, I was doing a VOD restore, and we decided to not replace the idle speed thermister. I had never heard of one going bad - figured at $35, not needed. Guess what failed and had my RPMs at 2500 until we learned to bypass the idle speed thermister by rewiring the idle ECU....

Also, on another note - DMCMidwest is installing a new transmission in my car right now - at least the "head portion" where all the hydraulics are located. While it's out, the trailing arm bolts have never been serviced in 30 years. Now, what's the failure rate of those bolts...? Pretty low. Than again, my car could look something like this if those 30 year old bolts break:

12271

My opinion: you've torn down the engine to the VOD - pop $40 and replace it. You will know it's done, and you won't have to come back underneath the manifold. At least, that's what I'd do for the peace of mind it'd give me :).

Ouch. Not good on those bolts. Yes, that's a smart thing to do...the bolts and the therm vacuum switch. I'll replace it. I think one thing that worries me is getting a replacement that actually is failing "out of the box". I've heard of other people ordering replacement "improved" oil sending units and having them fail right away. Which, by the way, I ordered the Borg Warner oil sending unit for $13 through Pep Boys today. They carry a lifetime limited warranty on it, too. For anyone interested, it's Borg Warner part number S769.

So on the idle speed thermister...I looked up the part on the exploded diagram and I can't really figure out where that one's installed. Where exactly is that one located? I think I'll go ahead and install that one too just to be on the safe side. Good idea? Kind of know the answer to that one...

Thanks again, guys!

Alex

[WelmoedJ]

So on the idle speed thermister...I looked up the part on the exploded diagram and I can't really figure out where that one's installed. Where exactly is that one located? I think I'll go ahead and install that one too just to be on the safe side. Good idea? Kind of know the answer to that one...
Alex

Right hand side of the Y pipe (the one with two connections on it).

DeLorean03: Would love to see the diagram for by-passing that idel speed thermister, could save a lot of trouble and time in the future if this one fails.

[Bitsyncmaster]

12270

Hi all. How often do these need to be replaced? DaveT, DaveM, and DaveMidwest, I'm sure you guys have a good idea. I'd appreciate anyone's opinion. I have the intake manifold off right now and in the middle of detailing/POR15 the valley along with a slew of other things so there would be no better time than now to replace it if it's a common failure point. But at $40, I'd rather get some opinions first. The only thing that was funky at the time I broke my engine down was I had a high idle (about 1000) once warmed up. Could this have been the issue, aside from most likely a vacuum leak?

Thanks.

Alex

When the idle thermistor is open you get and idle RPM around 2500. The idle thermistor sets how much "throttle" the engine gets when you start the car from cold to hot. The thermistor does not really affect idle RPM except for an open circuit.

Now if your starting the at at 30 deg blow 0.....the thermistor may look like an open circuit.

[David T]

The failure rate on the vacuum switch is very low. What is more common is bad hoses or improperly routed hoses, especially on the solenoid. The idle thermister does fail more commonly but there is also a failure rate of new parts. I would test the idle thermister and if it checks out good I would just reuse the old one. It isn't terrible to have to go back in to change it, just difficult. Definitely change the vacuum hoses, just be sure you do it correctly!
David Teitelbaum

[Stainless]

While it's out, the trailing arm bolts have never been serviced in 30 years. Now, what's the failure rate of those bolts...? Pretty low. Than again, my car could look something like this if those 30 year old bolts break:

12271



Wait... I thought it was established that this wreck was not caused by failed trailing arm bolts... http://dmcnews.com/Techsection/rollover.html

Or were you just saying that your car could look like that if yours failed?

[kings1527]

Right hand side of the Y pipe (the one with two connections on it).

DeLorean03: Would love to see the diagram for by-passing that idel speed thermister, could save a lot of trouble and time in the future if this one fails.

Oh, that one...the one that I just took off.:facepalm: Thanks.

I second the vote on the diagram for bypassing the thermister.


When the idle thermistor is open you get and idle RPM around 2500. The idle thermistor sets how much "throttle" the engine gets when you start the car from cold to hot. The thermistor does not really affect idle RPM except for an open circuit.

Now if your starting the at at 30 deg blow 0.....the thermistor may look like an open circuit.

Thanks Dave.


The failure rate on the vacuum switch is very low. What is more common is bad hoses or improperly routed hoses, especially on the solenoid. The idle thermister does fail more commonly but there is also a failure rate of new parts. I would test the idle thermister and if it checks out good I would just reuse the old one. It isn't terrible to have to go back in to change it, just difficult. Definitely change the vacuum hoses, just be sure you do it correctly!
David Teitelbaum

Thanks DaveT, that's exactly what I was looking for.

Alex

[DeLorean03]

Wait... I thought it was established that this wreck was not caused by failed trailing arm bolts... http://dmcnews.com/Techsection/rollover.html

Or were you just saying that your car could look like that if yours failed?

You know, now that you say that, I have heard the same thing - that it was NOT due to breaking TABs. We have; however, had a couple cases here at DMCTalk (back in the old days), where an owner had the tabs break at 5mph in the parking lot, and , well....boy they sure were happy it was at 5mph and not 55 mph.



DeLorean03: Would love to see the diagram for by-passing that idel speed thermister, could save a lot of trouble and time in the future if this one fails.

Here you go, WelmoedJ. What you want to do is take the wires for socket # 9 and 11, snip them and twist them together. That will eliminate the thermister. I will take a picture of my car's setup when I get it back from DMCMidwest in late August/early September to verify. I say that because I'm darn sure the two wires I twisted together were nearly right next to each other - not diagonally across from each other. The diagram picture could simply be inaccurate in terms of wire placement - always can look at the colors of the wires in your car to be totally safe.

Another thing you could do to "make sure" it's a bad idle speed thermister, if your car is idling at 2500, is create a jumper wire with alligator clips on each end. The idle speed ECU conveniently has breaks of wire with no insulated sheathing, so you could use your alligator clipped jumper to bypass the thermister, and if the idle goes low, you know it's bad. Once I did that, snip snip went my wires, and goodbye went my idle speed thermister forever.

http://dmctalk.org/attachment.php?attachmentid=103&d=1306264530

[DeLorean03]

Kings, here's a link you will love:

http://specialtauto.com/delorean-parts/reference-materials.html

John took some time to give us a VERY awesome reference page there. I have spent MANY hours going over his entire website - EXCELLENT pictures not just of references like that page, but just pictures of JUST the parts too. I mean, seriously, if you've never seen a DeLorean front vs rear wheel bearing, how will you know which to get - you know?