Loping idle then stall...uh oh
05-18-2012, 08:22 PM
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Loping idle then stall...uh oh
So I have had my 87 S4 for a about 3 weeks and it runs great. The weather hasn't been the greatest with a good amount of rain but I have had it out a handful of times. The past couple of days have been beautiful and I brought it to work for 3 days in a row with no problems. I was just about to head out to a buddies house for a bbq and the car started with no problems but the idle was loping. The tach stayed pretty much where it should be but the oil gauge was jumping up 1 bar and dropping with the lope. I put it in 3 and was creeping out of the garage when it ran real rough and died.....great! It started right back up but it was doing the same thing so I backed it back in as I'm running late and figured I'd just take my DD (even though I took 5 mins to write this 
). Any ideas what I should look at tomorrow? I did a really quick search but didn't come up with anything but I wil search better later when I'm home.
The oil gauge is at 4 and dropping to 3 and back to 4 as the idle lopes.
Man! It's beautiful out and was really looking forward to taking the Porsche.
Thanks guys.
). Any ideas what I should look at tomorrow? I did a really quick search but didn't come up with anything but I wil search better later when I'm home.The oil gauge is at 4 and dropping to 3 and back to 4 as the idle lopes.
Man! It's beautiful out and was really looking forward to taking the Porsche.
Thanks guys.
Last edited by -GDG-; 05-18-2012 at 08:59 PM. Reason: addition
05-19-2012, 12:41 AM
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The first thing I would check for is 1/2 of the engine not firing due to an ignition system problem (e.g. Bad coil, bad coil wire, bad coil amp)
I would check for this first because IF 1/2 the motor is not firing, then half of the fuel is igniting in the cats. See threads on cat fires, IMS system for '89+, and checking for spark and fuel.
Don't drive the car until you know your not dumping fuel into the exhaust!
I would check for this first because IF 1/2 the motor is not firing, then half of the fuel is igniting in the cats. See threads on cat fires, IMS system for '89+, and checking for spark and fuel.
Don't drive the car until you know your not dumping fuel into the exhaust!
05-19-2012, 01:09 AM
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Agreed with Dave.....sounds like only 1/2 the engine is working.....so check the coil-dist on each side....I don't think 87 S4's have limp home 4 cyl mode....like the 89+ do...
05-19-2012, 03:30 AM
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I did struggle for months with a similar problem of looping idle and then engine stall (and oil pressure on the gauge jumping with the idle); it could be a number of potential causes, vacuum leak, LH Jetronic,MAF etc. I recommend to run an Electronic dagnosis with the Bosch Hammer set and check knock sensors, first and foremost. Mines were half eaten by rodents who managed to "house themselves" under the intake manifold.
05-19-2012, 05:55 AM
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The fact that the idle is hunting is a good sign as it means the LH ECU is trying to get the idle loop in lock and also that the ISV is responding to the LH ECU.
It also suggests that the closed throttle switch is working OK, as it is this that activates the idle control loop.
If the idle was bad from a cold start that tends to rule out the O2 sensor. It it only went bad once the engine water temp was up to normal, then O2 sensor could be bad.
The problem could well be igntion. Check if there is a spark from both distributors and let us know what you find.
You can't use a diagnostic tester on your 87 car until you swap the EPROMs in the LH and EZK ECU for the MY88 diagnostic versions. I can supply these.
It also suggests that the closed throttle switch is working OK, as it is this that activates the idle control loop.
If the idle was bad from a cold start that tends to rule out the O2 sensor. It it only went bad once the engine water temp was up to normal, then O2 sensor could be bad.
The problem could well be igntion. Check if there is a spark from both distributors and let us know what you find.
You can't use a diagnostic tester on your 87 car until you swap the EPROMs in the LH and EZK ECU for the MY88 diagnostic versions. I can supply these.
05-19-2012, 08:50 AM
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I had this issue with my 87s4 and after a LOT of diagnostics and part replacements, it was the LH ECU causing the symptoms you describe.
The crank sensor and Hall sensor connectors were crumbled plastic, and caused a lot of issues but not the symptoms you describe. Try swapping your LH into a car that is known to be running correctly.
The crank sensor and Hall sensor connectors were crumbled plastic, and caused a lot of issues but not the symptoms you describe. Try swapping your LH into a car that is known to be running correctly.
05-19-2012, 10:11 AM
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Hi guys,
I really do appreciate your help. I have an hour or so to look at it this afternoon & I will check the distributors. Very strange, the car ran great yesterday and sat for about 2hrs and now this. Trouble shooting a little hard right now as I don't have the shop manuals yet but I will do what I can. I have an appointment at PowerTech (local P shop) for Monday that I scheduled 3weeks ago (I guess they're that busy!) so right now I am just hoping to get the car there without incident or FIRE! Damn that's scary.
Thanks again for the help!
I really do appreciate your help. I have an hour or so to look at it this afternoon & I will check the distributors. Very strange, the car ran great yesterday and sat for about 2hrs and now this. Trouble shooting a little hard right now as I don't have the shop manuals yet but I will do what I can. I have an appointment at PowerTech (local P shop) for Monday that I scheduled 3weeks ago (I guess they're that busy!) so right now I am just hoping to get the car there without incident or FIRE! Damn that's scary.
Thanks again for the help!
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05-19-2012, 10:13 AM
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The fact that the idle is hunting is a good sign as it means the LH ECU is trying to get the idle loop in lock and also that the ISV is responding to the LH ECU.
It also suggests that the closed throttle switch is working OK, as it is this that activates the idle control loop.
If the idle was bad from a cold start that tends to rule out the O2 sensor. It it only went bad once the engine water temp was up to normal, then O2 sensor could be bad.
The problem could well be igntion. Check if there is a spark from both distributors and let us know what you find.
You can't use a diagnostic tester on your 87 car until you swap the EPROMs in the LH and EZK ECU for the MY88 diagnostic versions. I can supply these.
It also suggests that the closed throttle switch is working OK, as it is this that activates the idle control loop.
If the idle was bad from a cold start that tends to rule out the O2 sensor. It it only went bad once the engine water temp was up to normal, then O2 sensor could be bad.
The problem could well be igntion. Check if there is a spark from both distributors and let us know what you find.
You can't use a diagnostic tester on your 87 car until you swap the EPROMs in the LH and EZK ECU for the MY88 diagnostic versions. I can supply these.
I'll do a search on the MY88 version that you can supply. Can you elaborate for a newbie? Thanks a lot!
05-19-2012, 02:28 PM
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The MY cars were wired for diagnostics, and have the conector ready for the tester to plug into.
It seems that porsche hadn't developed the tester software in time for the product launch and it wasn't introduced until the MY88 which like all Porsche MY actually started in the previous August of 1987.
Both LH and EZK ECUs have plug in operating software EPROMs and it is easy to swap them for the later chips with the diagnostics. I can supply or someone with a chip burner will make a pair for you, I can email them the required files.
MY87 & 88 use a 12way diagnostic connector, different to the 89 and later cars.
It is a two row rectangular connector, which is mounted on the plate that holds the LH and EZK ECUs.
It seems that porsche hadn't developed the tester software in time for the product launch and it wasn't introduced until the MY88 which like all Porsche MY actually started in the previous August of 1987.
Both LH and EZK ECUs have plug in operating software EPROMs and it is easy to swap them for the later chips with the diagnostics. I can supply or someone with a chip burner will make a pair for you, I can email them the required files.
MY87 & 88 use a 12way diagnostic connector, different to the 89 and later cars.
It is a two row rectangular connector, which is mounted on the plate that holds the LH and EZK ECUs.
