Author Topic: I'm trying to track down a rough idle problem on my 87 GN  (Read 704 times)

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Offline john robertson

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I'm trying to track down a rough idle problem on my 87 GN
« on: January 12 2018, 06:47:31 PM »
Hello all. I'm having an idle problem. On startup, it may idle smoothly for a couple minutes and then the idle rises and falls, and gets somewhat jerky and then may smooth out. Once into normal driving, all seems well. I have swapped out (I have another turbo car) the ign. mod./coilpak, the maf, ECM, changed chips so far. I've unplugged the O2 sensor. None of this had any positive effect.

I finally got some improvement by changing the TPS. It idled smoothly for longer periods although with some, but less than before, occasional idle fluctuating. If I unplug the TPS it idles perfect. Is this typical? Though it does seem overall better with a different TPS installed, I still think it's not the basic problem.

I unplugged the cam sensor. After doing so, about 30 seconds later the car stalled out. I tried again, and it stalled out quicker. I was under the impression it would just continue to run. What might that indicate if unplugging the cam sensor caused it to stall?  Thanks in advance for any and all suggestions.

Offline Steve Wood

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #1 on: January 12 2018, 07:02:11 PM »
John, are you running factory ignition modules, or, are they aftermarket.  My cars run perfectly on factory modules when unplugged.  Of course, I get the check engine light.

I never tried unplugging an aftermarket module.

Just for the heck of it, you might try loosening the cam sensor and rotate it about 1/8" CCW and see if that makes a difference.  While you are at it, Check how much vertical play you have in the cam sensor cap.

Is the car warming up properly after start up as per the scantool and do you have any data logs taken during the warm up to show us?
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Offline TexasT

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #2 on: January 12 2018, 07:06:40 PM »
Welcome to the site. I'm sure Steve and the gang will have it sorted out or at least some suggestions of how to do it shortly.
Rich

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Offline john robertson

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #3 on: January 12 2018, 07:38:48 PM »
John, are you running factory ignition modules, or, are they aftermarket.  My cars run perfectly on factory modules when unplugged.  Of course, I get the check engine light.

I never tried unplugging an aftermarket module.

Just for the heck of it, you might try loosening the cam sensor and rotate it about 1/8" CCW and see if that makes a difference.  While you are at it, Check how much vertical play you have in the cam sensor cap.

Is the car warming up properly after start up as per the scantool and do you have any data logs taken during the warm up to show us?

Hello!  :icon_smile: Yes, the TPS is an AC, or that's what it says, and the cam sensor is not aftermarket as far as I know. But as I said, it stalls after unplugging the cam sensor. Does that mean anything? And when unplugging the TPS, there is absolutely no idle variation.

I thought maybe I might be too close to a position with the cam sensor that might cause my idle problem, but I didn't get that far -- to loosen it and move it a little, so I will do that tomorrow.

Car is performing fine after warmup, though I've been staying out of WOT for fear of back fires. Of course I have a scanmaster but nothing more sophisticated.  Thanks!

Offline Forzfed

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #4 on: January 12 2018, 08:15:01 PM »
How are your injectors?

Offline john robertson

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #5 on: January 12 2018, 08:26:25 PM »
I think they're OK. I have 50s I installed years ago, before there were 60s. As I said, it will run smooth, so if it was something like that, it wouldn't ever run smooth. This is just one of those problems that starts, out of the blue. I'm hoping it goes away the same way.  :icon_smile:

Offline Steve Wood

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #6 on: January 12 2018, 10:16:09 PM »
John, I was asking about your ignition modules...are they factory from AC or are they made by someone like Wells or Borg Warner.  I am trying to understand why the car will not idle with the cam sensor unplugged.

With regard to rotating the cam sensor slightly CCW, some times, the wear in the cam sensor shaft will allow the wheel under the cap to rise higher as the engine turns and as it rises higher, it can sometimes move slightly out of the adjustment window.

I asked about the coolant temp reported on the scanmaster to see if it was reading correctly in order for the ecm to invoke closed loop...I guess that suggests to ask if the light on the scanmaster indicates closed loop when it warms up.

Further, set your scanmaster to tps and see if it stays at 0.46v or less.  I prefer no more than 0.44 to give myself a little margin for error...it should be in the window of 0.38 to 0.46 volts to properly command the ecm to be in the idle mode.

Might as well ask if the iac's are somewhere between 10-30 counts when the car is warm, shifter in park, and the AC switched off. 

Coujple of more questions...wh at chip are you running in the ecm?  Is it a modern one like from TT?

What are your O2 numbers at idle?  With a modern chip that provides an open loop idle, the numbers should be pretty steady near .800 at idle in Park after the engine has run a couple of minutes so that the O2 sensor has warmed up.

Steve Wood

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Offline john robertson

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #7 on: January 13 2018, 06:56:28 AM »
Sorry on the misunderstandi ng on the "modules". When I swapped the ignition module/coilpak from my other turbo car to my GN, it is, in fact, an aftermarket module, so I guess that's why the car shuts off after unplugging the cam sensor. I'll be switching back the modules first thing todaythe other being a Delco. Also, I have one of those Casper's cam sensor caps, brand new, that I have never installed. Now may be a good time to put it on. Steve, would you do that or do you recommend loosening the cam sensor and moving it ccw?

The car does warm up properly and readings on the scanmaster are in order. It does go to closed loop normally. I'm not familiar with a light on the scanmaster indicating closed loop. Also, I'm pretty sure TPS reading sits on .042 and doesn't change unless I press on the gas, but I'll double check.

I upgraded my TT chip less than a year ago, I think it's 5.7, for more fuel and yes, the 02s are reading in the 800s on startup. I'll verify that today to be sure. I'll be looking at it in a couple hours.

It seems this problem presented itself after a cold weather snap down here in Florida. I didn't go anywhere in the GN for over a week and when I resumed driving it, that's when I noticed the erratic idle. Thanks again.

Offline Steve Wood

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #8 on: January 13 2018, 08:54:22 AM »
If the other car ran properly with that ignition module, I doubt it is the problem. I was just curious why it would not idle unplugged at the can sensor.

Try moving the cam sensor before swapping the caps ☺️
Steve Wood

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Offline ULYCYC

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #9 on: January 13 2018, 09:12:15 AM »
Make sure your cam sensor is not 180degs out.  this can cause stalling with sensor is unplugged when running. Also a stretched timing chain can cause many issues that points you looking in the wrong direction
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Offline john robertson

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #10 on: January 14 2018, 05:38:10 PM »

Sorry I didn't get back sooner, but something came up. Seems to be OK now. As I said, a different TPS seemed to make a difference. Also, I think I moved the cam sensor very slightly CCW. It was tight and I used a screw driver to pry against the cap retainer screw boss gently. Figured I'd just start it up and there was nothing objectionable at idle. Thanks all very much for your input.

Offline Steve Wood

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Re: I'm trying to track down a rough idle problem on my 87 GN
« Reply #11 on: January 14 2018, 07:44:56 PM »
Good deal :)
Steve Wood

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