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914World.com _ 914World Garage _ CHT failure and bad idle linked?

Posted by: Skid Apr 10 2005, 12:47 PM

My car has a 1.8 FI that has, up to this week, always run great and idled perfectly. The problem has just started happening the last couple of days. If I drive the car more than a couple of miles, it starts idling bad and if it kills, it wont start back up. I read that my head temp sensor could be causing the starting issue, but could it also cause the bad idle? If not the CHT, what else could be wrong?

Posted by: ArtechnikA Apr 10 2005, 01:47 PM

yes.

fix that first, if it's bad, and then we'll take on whatever symptoms is has after that.

without a valid CHT signal, any other attempts at diagnosis are useless.

Posted by: Mueller Apr 10 2005, 01:50 PM

there is also a chance that the CHT is good, but it is not as tight as it needs to be in the head....do a search for L-Jet diagnostics/help and you should be able to find the specs for the CHT to look for with your digital volt meter.

Posted by: brownaar Apr 10 2005, 03:49 PM

I had similar problems with a loose CHT sensor. Ran great, ran like crap, tightened loose sensor, ran great. My car starting doing the same things again today, so I've got to go check the damn sensor once she cools down.

Aaron

Posted by: 3d914 Apr 10 2005, 05:20 PM

Skid,

Keep in mind that if the sensor is tight, there could still be problems with it. You can do static tests on it to see if continuity is good, but because of where its located thats very difficult while the car is running.

I had similar problem, and static tests showed nothing. It wasn't until I installed a new fuel injection system (for other reasons) that I was able to actually see the sensor feedback while the car was running. Every little bit the sensor would drop out. If I shut if off and did a static test - it checked out OK. Finally pulled the sensor and found that the lead into the top of the sensor was very weak - causing a sporatic signal.

Don't spend too much time on this if it tests OK statically, but fails again when running. For the $12 they cost, its easier to pull it and replace it.

Just my $0.02

Enjoy!

Posted by: Skid Apr 20 2005, 11:57 AM

Update: Sorry, long strory.

Ok, last week, I decided that I should first try to replace the CHT. After putting in the new CHT, I did not notice a change in the wandering idle but I figured that the killing problem was fixed. Took the car to an auto-x and after one run, it killed and once again wouldnt start. Luckily there was a mechanic there who knew quite a bit about 914's, he quickly diagnosed the problem as bad points and while we waited, he ran to his shop to grab some new ones. After installing the new points and checking to make sure they were working with a cool computer thing, the car fired right up. I was able to take it out and get three runs. The car seemed to have more power than ever. During a break in the racing I took the car to get some more gas as I was running low. The car got three more passes on the new gas and after being shut down for a few minutes, wouldnt start again. The mechanic said that it was getting spark and the problem must have been fuel related. He had to leave so he could not help figure out the problem. I thought that the car was flooded so I floored it and tried starting it again. She fired up and although not idling very well at first, she ran just as good for my first run. During my second and third run though, when I would let the RPM drop, the motor would stutter and almost kill before responding to throttle input. It did this several times and finally killed at the end making me coast the last 100 feet of the race. After pushing the car out of the way, it once again would not start. after sitting for a few hours, she fired right up again.

What is wrong with my car, I originally thought it was the CHT, then the points, maybe I got some bad fuel, I just dont know.

Anyone have any good ideas because I am stumped.

Posted by: brownaar Apr 20 2005, 01:00 PM

Skid,
If you get this one figured out, let me know. Mine is doing similar things. Starts fine when cold, won't idle/hard to start when warm. I suspect mine is CHT since the resistance value is high at 6.5 Kohms when cold, but I can't change mine out without pulling the head since some prior individual rethreaded the CHT slot with coarse threads. Anyway, I'd love to know if there is another possible fix, as I don't relish pulling the head. My next step is to ground out or put a variable potentiometer in the CHT circuit with the hot engine and see if it makes a difference.

Aaron

Posted by: Travis Neff Apr 20 2005, 01:28 PM

Check Brad Anders site http://members.rennlist.com/pbanders He states there is a problem with 2.0 injection that when the car is hot started it tries to dump a ton of fuel into the engine.

My car does the same thing when hot, it is a 72 1.7

Posted by: Skid Apr 20 2005, 10:22 PM

The car seemed to run great after the new points were put in and it had a consistent idle. It was only after I got gas that it started acting wierd, do you think I may have gotten bad gas or does this sound like the same problem as before?

Posted by: Skid Apr 20 2005, 10:24 PM

QUOTE
Check Brad Anders site http://members.rennlist.com/pbanders He states there is a problem with 2.0 injection that when the car is hot started it tries to dump a ton of fuel into the engine.

Does that only apply to D-jetronic injection because I believe that 1.8's have L-jetronic?

Posted by: Travis Neff Apr 20 2005, 10:28 PM

It's official, I'm stooopid - should have read a little more carefully

Posted by: Skid Apr 20 2005, 10:42 PM

QUOTE
It's official, I'm stooopid

No, at least you are trying

I just stare at my car with a blank look on my face
Hopefully someone will have a solution.

Posted by: nebreitling Apr 20 2005, 11:13 PM

we can't give you a "solution" based on that. it could be about 30 different things, certainly including the CHT. this is the time to pull up your sleeves and dig in. begin going through the ignition and then the FI methodically until you find the problem. you can do it. it will just take some patience. and we're always here to help if you have any specific questions. n

Posted by: 3d914 Apr 22 2005, 08:00 AM

Skid,

Another thing to consider is the possible failure of the Cold Start valve. This dumps extra fuel when cold to add startup. If its leaking fuel, it will flood a warm engine on startup. Its located at the back of the plenum on the drivers side. Same place the Aux Air Regulator air hose routes to.

THe ECU (Injection contoller) could also be a factor, but this will require a testor (still available from Pelican Parts I believe). I had two of these for my 2.0L and both were causing rich idle.

Keep at it.