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Rough idle and rough running when engine sits a gets heat soaked


Justanub
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Posted Today, 09:58 PM

Help. My mechanic gave up, told me to find a Pathy expert. And he has had 3!

 

Here is what I am dealing with. 1991, 106,000 miles, automatic. I noticed a slight miss a few weeks ago driving in traffic. So slight I first thought it was the road. Stopped at a store for 15 minutes, came out, and was difficult to start. Idled rough, couldn't make more than 30 mph but I was able to drive 4 miles or so and got home. I let it cool and she started right up and ran just fine.

 

Now I know how to make it fail. I drive 10 - 20 minutes on a hot day - 90 plus. Park it for 10-15 minutes. Then it will barely startand idles rough. Can't really drive it anywhere. Let it cool 35 minutes with the hood up and she starts right up. Here is the weird part, I can drive it after it is cool and It doesn't get worse, just a slight miss or two.

 

The problem does not come back until I let it sit and heat soak the engine compartment.

 

Work so far - new fuel pump, new fuel filter, new disti cap and rotor. No error codes. She had erratic fuel pressure so that is why we started there. The fuel pressure is now good. My mechanic replaced the coil and coil wire but it didn't fix it so the old one is back on.

 

My theory is that something is getting heat soaked under the hood and causing the problem. But what?

 

I am thinking I can get the engine hot again, the start cooling thing with ice and compresses air and see if she runs again, but where to start? Coolant sensor? Wiring harness?

 

Has anyone seen this behavior in there Pathy?

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Thanks Slartibartfast! Would a camshaft position sensor start to fail and not throw out any codes? The mechanic said he "scoped" the engine with his shop sensor while it running bad but did not see any errors, so I don't think he was looking at the ECU. I have gone through the ECU diagnostics but that was after it had been turned off and cooled down.

 

Should I run the diagnostics on the ECU while the truck in on and running rough?

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Slartibartfast - taking your hunch and searching on camshaft position sensor I found this discussion from WarthogWD21 in 2012. JamesRich described his symptoms which mirror mine - he said he replaced the crank angle sensor - but I don't see such a thing in the shop manual, so I am thinking he means camshaft position sensor.

 

 

If the engine was cold it would start right up. If I shut it off when the engine was warm and wait 5 minutes or more I was screwed and had to wait till it cooled off. What was weird though once it was running it never quit. It did this to me when I was hunting. I slept in the next morning and she started right up. I loaded up and drove an hour and a half back home with no problems. Shut it off at home and it started right back up. Gave it 5 minutes and it wouldn't start. Changed the crank angle sensor and everything was good again. Drove back to the hunting camp.

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Slartibartfast - taking your hunch and searching on camshaft position sensor I found this discussion from WarthogWD21 in 2012. JamesRich described his symptoms which mirror mine - he said he replaced the crank angle sensor - but I don't see such a thing in the shop manual, so I am thinking he means camshaft position sensor.

 

 

If the engine was cold it would start right up. If I shut it off when the engine was warm and wait 5 minutes or more I was screwed and had to wait till it cooled off. What was weird though once it was running it never quit. It did this to me when I was hunting. I slept in the next morning and she started right up. I loaded up and drove an hour and a half back home with no problems. Shut it off at home and it started right back up. Gave it 5 minutes and it wouldn't start. Changed the crank angle sensor and everything was good again. Drove back to the hunting camp.

Sounds like the same part to me. Hopefully that solves it for ya!

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Yep, there's just the one sensor. The service manual calls it the camshaft position sensor (which makes sense, seeing as how the dizzy is geared off the camshaft).

 

There is a code for this sensor (11, "Camshaft Position Sensor Circuit"), but the ECU only throws that code if "Either the 1° or 120° signal is not entered for the first few seconds during engine cranking" or "Either 1° or 120° signal is not input often enough while the engine speed is higher than the specified RPM". So, if the sensor doesn't say anything at all, or keeps missing a beat, it'll code. Other than that, the ECU will believe just about anything the sensor throws at it, and adjust the fuel injection and spark timing accordingly. I'd throw a timing light on it when it's acting up and see if the marks jump all over the place.

 

I've heard of guys replacing just the sensor and being fine, but I've also heard that the bushings in the distributor itself can fail so it doesn't spin freely or the bushing dust gets into the sensor wheel or something. I haven't dealt with this issue on mine so that's about all I know about it.

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I am going to do some testing this weekend. I'll see if I can get it hot enough to act up then cool down just the distributor with some cool water and a shot of compressed air. I bet money it will run normally.

 

There are 5 wd21s at the local Upull. I might go get one and see if it fixes the issue. But since all those will be higher miliage engines than mine I see a new distributor in my future.

 

I let you know how it comes out.

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Yep, there's just the one sensor. The service manual calls it the camshaft position sensor (which makes sense, seeing as how the dizzy is geared off the camshaft).

 

There is a code for this sensor (11, "Camshaft Position Sensor Circuit"), but the ECU only throws that code if "Either the 1° or 120° signal is not entered for the first few seconds during engine cranking" or "Either 1° or 120° signal is not input often enough while the engine speed is higher than the specified RPM". So, if the sensor doesn't say anything at all, or keeps missing a beat, it'll code. Other than that, the ECU will believe just about anything the sensor throws at it, and adjust the fuel injection and spark timing accordingly. I'd throw a timing light on it when it's acting up and see if the marks jump all over the place.

 

I've heard of guys replacing just the sensor and being fine, but I've also heard that the bushings in the distributor itself can fail so it doesn't spin freely or the bushing dust gets into the sensor wheel or something. I haven't dealt with this issue on mine so that's about all I know about it.

The camshaft position sensor in my 240sx went bad and you could clearly feel it while under load. If it's going bad you'll be able to clearly see it if you hook the test lead of a timing light to the main wire coming from the ignition coil.

 

And to add to this it's the bearing under the sensor that goes bad and allows there to be enough play in the unit to allow the degree wheel to come in contact with the sensor "eye". Once this is damaged there's no going back. You can get just the bearing and the sensor separately and replace them yourself or you can get a new/reman distributor. I've always just rebuilt mine and called it a day, I did this when I got my frontier. It was so bad you could see the debris from the bearing all over the inside.

 

Sent from my SM-G920V using Tapatalk

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I went to the local U-Pull yard and grabbed a dizzy out of a 186k 1991. I put it in a couple of days ago and she is driving just fine. I have not been able to replicate the rough running after getting her up to operating temperatures and then letting her sit.

 

The issue was really a problem when the outside temp was in the 90s and it has not been that hot this week, but I really think the issue was the Camshaft Position Sensor.

 

Slartibartfast nailed it. Thank you!

 

One more question - I think I should buy a new dizzy. Any recommendations? I hear there is cheap Asian imported parts I should avoid, but it is hard to tell on the online sites who the manufacturers are.

 

 

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