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Dropping RPM when cold
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=86881		 Page 1 of 1
Author:  masey22 [ Mon Jul 17, 2017 10:01 am ]
Post subject:  Dropping RPM when cold
I can't believe I have been a member since 2013 and never made a post.
I suppose its never too late to start. I am having a couple of, probably unrelated, issues.

1) As the post title implies, on a cold start the rpm falls by 200-300, about a dozen times. and then smooths out. I have already replaced the fuel filter, and it has a new filter head on it less than 20k ago. But a I am still not convinced it is not something fueling related. The other thought is that it might be a bad glow plug? or two?

2) I have a ETC light that goes on and off throughout driving, I have read multiple people's posts about loose wiring. but there doesn't seem to be any.

3) My engine temp gauge will shoot up to like 50% beyond hot and then fall back down to something more normal and keep cycling like this until it is hot. I am assuming this is probably just a bad Coolant temp sensor. But thought I would ask for other ideas before I dig into it.

The first two happened during a 600 mile trip, and the third after the oil change, and fuel filter replacement that followed the trip. I also recently replaced my blown out boost hoses with silicone ones from IDparts.

The jeep is completely stock, with 156k.
Author:  mass-hole [ Mon Jul 17, 2017 10:56 am ]
Post subject:  Dropping RPM when cold
So the tune has a "high idle" feature based on coolant temp and ambient pressure(altitude). If your coolant temp sensor is messed up, which it sounds like it is, then that's likely what is going on. My jeep will idle closer to 1000 rpm when cold in the winter, before it settles at the normal 760. The other issue is it's possible you have air or gasses trapped in the thermostat housing. May be worth trying to burp the radiator to see what comes out. Or, maybe there is a wiring issue as you said which is causing the temp reading to fluctuate and set off the light.


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Author:  papaindigo [ Mon Jul 17, 2017 3:02 pm ]
Post subject:  Re: Dropping RPM when cold
Are you running an OEM thermostat and if so how old in miles is it? I've seen temp. cycling on other vehicles but not running way above normal, for the CRD that's essentially straight up. My 1993 Dodge D250 Cummins did it all the time and the cycle reflected the thermostat opening and closing as the engine warmed up until it settled down once the engine was warm. Offhand my guess is your thermostat is about shot and you may have a bad thermostat housing temp sending unit.
Author:  masey22 [ Mon Jul 17, 2017 3:18 pm ]
Post subject:  Re: Dropping RPM when cold
It has less than 30k on it.
So I would be surprised if it was bad.
Also it is cycling way to fast for me to really think that that could be issue, it goes from below the first tick to well past hot in less than a second.
Author:  mass-hole [ Mon Jul 17, 2017 3:53 pm ]
Post subject:  Re: Dropping RPM when cold
masey22 wrote:
It has less than 30k on it.
So I would be surprised if it was bad.
Also it is cycling way to fast for me to really think that that could be issue, it goes from below the first tick to well past hot in less than a second.


Yeah I bought my Jeep with 42k on it and it was already dead. I bought it in August though so I didnt realize it till December or so.
Author:  GordnadoCRD [ Mon Jul 17, 2017 4:24 pm ]
Post subject:  Re: Dropping RPM when cold
masey22 wrote:
It has less than 30k on it.
So I would be surprised if it was bad.
Also it is cycling way to fast for me to really think that that could be issue, it goes from below the first tick to well past hot in less than a second.

This, To me, sounds more like compromised insulation allowing an intermittent ground.

To your issue #1, mine does the same thing when the Viscous Heater clutch, and AC (defrost) clutch, cycle. Once warmed up, that stops though.
Author:  masey22 [ Tue Aug 01, 2017 9:25 am ]
Post subject:  Re: Dropping RPM when cold
GordnadoCRD wrote:
masey22 wrote:
It has less than 30k on it.
So I would be surprised if it was bad.
Also it is cycling way to fast for me to really think that that could be issue, it goes from below the first tick to well past hot in less than a second.

This, To me, sounds more like compromised insulation allowing an intermittent ground.

To your issue #1, mine does the same thing when the Viscous Heater clutch, and AC (defrost) clutch, cycle. Once warmed up, that stops though.



To give some closure to this thread, I finally have the issue sorted out. It was compromised insulation on a red and purple wire running from the main harness across the top of the head to the passenger side of the engine bay.
I think what happened was that I had been running the jeep without the engine cover on for most of the time that I had it, and then when I moved into my new house I just threw it back on and that wire got pinned in a peculiar way and wore through the insulation and grounded.

So thanks GordnadoCRD for the idea!
Author:  GordnadoCRD [ Tue Aug 01, 2017 5:36 pm ]
Post subject:  Re: Dropping RPM when cold
Glad we could help, that you found the issue, and it's fixed, and it wasn't something expensive!
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