Rough idle/ misfire on an 85

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Ralph in Socal
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Re: Rough idle/ misfire on an 85

Post by Ralph in Socal »

Probe the ecu connector at different engine temperatures. Doesn’t have to be running but key may have to be ON. Terminal #13 is for the coolant temp sensor. Make sure that you get the same resistance values at the sensor as you do at the ecu. I had a car that exhibited the same symptoms and turned out to be a broken wire to the cts. Good luck

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Re: Rough idle/ misfire on an 85

Post by Steve M6 »

Been looking at this thread with some interest, and one thing seems to be overlooked. Secondary Ignition problems.

I have had experience with spark plug wires deteriorating to the point where the engine seems to be due for an overhaul. They could be your problem. Defective Spark plugs could also be contributing.

I have also had a problem with the factory ignition coil on my '88 BMW R100RT motorcycle. It would quit for no apparent reason (engine would abruptly stop), then began to work again when it cooled off. It turned out it had a cracked insulator, and I found out later that that is not an uncommon problem.

Hope this helps...
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

I'm sure a new coil is in order, it's probably overdue for one. However all the plug wires and plugs are brand new and all have been tested for resistance
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Re: Rough idle/ misfire on an 85

Post by Ralph in Socal »

You can test for fouled spark plugs while installed by resistance metering. I set my DMM to 2000k ohms (Max setting) and read between the plug terminal and the head. Any reading indicates a fouled plug

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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

New coil on, no change. Plugs are fine.
All I do know for sure is that whatever the issue is it is affecting all cylinders equally and pretty drastically. I can unplug any given plug wire and I see almost no change in the idle. I suppose I'll pull the injectors out this weekend to have them tested and cleaned.
Car sounds like it's got a serious cam once I get it to stay running, very choppy sounding and low idle.
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Re: Rough idle/ misfire on an 85

Post by Steve M6 »

I have had experience with an Audi 5000 that practically would not run due to a clogged catalytic converter. I have never experienced a cat failure on one of my BMWs, but that seems to be a distinct possibility, especially since you have tried to eliminate so many other potential causes.

It would be good to substitute a known good one, if that is possible, before spending the $$. Maybe just temporarily open up your exhaust and see what happens?
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Re: Rough idle/ misfire on an 85

Post by Ralph in Socal »

I think the ecu is simply going to full rich due to a failed sensor or interrupted signal. At this point I would probe the ecu connector for the various inputs like tps, O2, cts, and afm. The ETM that was linked earlier provides the terminals for the different sensors.

Basically, just because the sensor is good doesn’t mean the ecu is getting a signal

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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

Haven't posted any updates because I've been out of town. Just got around to pulling the injectors to have them cleaned this week and they are VERY dirty. All but one are filled with rust and dirt, I'll certainly be replacing the filter again after seeing them. Hoping like crazy that these are the cause of the issue, won't know for sure until they get back from the cleaners. Attached a picture of the CYL 1 injector, it seems like the they get dirtier from front to rear. Cyl 1 being the worst and 6 being the cleanest.
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

Another update, just got the injectors back and while they all resistance tested good I was told that they are all leaking and dripping no matter how many times they would clean them out, although they come back to life a little after running them through the machine, the shop advised that they are too far gone and need to be replaced. I've ordered a remanufactured set that will be in this week so hopefully that will be the end of the thread.
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

New injectors and fuel filter on the car annnddddd doing the exact same thing. Only real noticeable change is that there's a little burble to the exhaust now. Sounds like a pop or backfire tune when you let off the gas. I really have no idea where to look next with this thing. If anyone has any further input it would be appreciated
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Re: Rough idle/ misfire on an 85

Post by hornhospital »

I see nowhere that you tested the O2 sensor or it's connection with the ECU. If you have checked the O2 sensor and circuit, my apologies.

Your car displays every symptom of a failed O2 sensor. The ECU has defaulted to full rich, running the injectors wide open to keep the pistons from melting (better full rich than full lean).
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

The O2 sensor is new, but I haven't tested it to the ecu pin.
After doing some more scouring last night, I saw someone mention having a similar issue using the same brand distributor cap ( bremi ). It has a new bosch rotor but couldn't source the bosch cap locally at the time so I just went with the bremi. It seems plausible that the bremi cap is junk and somehow throwing off the Ignition timing. I can't shake the feeling that it feels like a timing issue and that could certainly be it. I've ordered a bosch cap and another new set of plugs, the wr9ls that seem to be the go to on this forum.
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

Starting probing connectors the other day. Found that the ecm was not getting signal from the O2 sensor due to a bad connection just outside the firewall in the engine bay. Cut the bad connectors off and soldered the connections together. Getting o2 sensor power into the ecm now, unfortunately the issue persists...
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

hornhospital wrote: Thu Aug 26, 2021 5:30 pm I see nowhere that you tested the O2 sensor or it's connection with the ECU. If you have checked the O2 sensor and circuit, my apologies.

Your car displays every symptom of a failed O2 sensor. The ECU has defaulted to full rich, running the injectors wide open to keep the pistons from melting (better full rich than full lean).
Thanks for the input, per my last post, everything tested out well except for the o2 sensor, which I repaired.
When testing the coolant temp sensor, I found a huge amount of resistance. Pulled the rubber boot that covers the connector and found a resistor ( tested it at 270 Ohms ) that looks to be poorly soldered inline. Is this a factory resistor? Weird that BMW would put a resistor inline when the CTS is basically a resistor. It seems more likely that someone wired it in at some point to compensate for a failing CTS.

I don't know how to post pictures on here otherwise I'd upload a pic of it.
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

hornhospital wrote: Thu Aug 26, 2021 5:30 pm I see nowhere that you tested the O2 sensor or it's connection with the ECU. If you have checked the O2 sensor and circuit, my apologies.

Your car displays every symptom of a failed O2 sensor. The ECU has defaulted to full rich, running the injectors wide open to keep the pistons from melting (better full rich than full lean).
Thanks for the input, per my last post, everything tested out well except for the o2 sensor, which I repaired.
When testing the coolant temp sensor, I found a huge amount of resistance. Pulled the rubber boot that covers the connector and found a resistor ( tested it at 270 Ohms ) that looks to be poorly soldered inline. Is this a factory resistor? Weird that BMW would put a resistor inline when the CTS is basically a resistor. It seems more likely that someone wired it in at some point to compensate for a failing CTS.

I don't know how to post pictures on here otherwise I'd upload a pic of it.
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Re: Rough idle/ misfire on an 85

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to upload a picture click the "attachment" tab below the text box area. There is a size limit so you may have to resize the file. HTH
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Re: Rough idle/ misfire on an 85

Post by Ralph in Socal »

The in-line resistor inside the boot of the cts is a dealer modification to address poor fuel quality in the 80s. It can be removed and the cts can go directly to the ecu. Proper resistance should be about 1000 ohms cold then less resistance as the engine gets hotter. Again, make sure the ecu is getting the same resistance value the sensor is providing. Good luck as you may have found the problem

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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

So I've removed the resistor on the CTS and soldered the wiring back together. It didn't change anything with the issue other than the fact that it took longer to warm up before it would stay Idling.
( also to add, after the repair I did verify the resistance was the exact same at the pin on the ecu connector as it was at the sensor itself )

Just to reiterate to those who are just hopping in to the thread, the cars problem is- Long crank on first start up, once it fires up it needs throttle applied for a minute or two or else it will stumble and die. Once it warms up it will idle low, with an obvious random misfire, sounds choppy like it has a large cam, the misfire becomes worse and more evident when it's put in gear.

So far I've rebuilt the trans, rebuilt the alternator, replaced the battery, cleaned the grounds, then replaced- O2 sensor, TPS, Fuel filter, Fuel pressure regulator, all 3 important relays, spark plugs (wr9ls gapped to .28), distributor, rotor, plug wires, coil, all 6 injectors, and adjusted valves on the cam side. Most everything is Bosch parts. Both fuel pumps are about a year old

I've tested all the main engine control sensors, and tested their connections to the ECM. Found the O2 sensor connection was bad and fixed it. Found a resistor with a poor connection inline at the CTS and removed it, fixed the wiring there as well.

Smoke tested for vacuum leaks, none found, engine will die when pulling the dipstick. The fuel pressure is a steady 37psi when running, 45psi when off. Compression shows 150 +- 5 psi on every cylinder. Timing appears to be set correctly as well.

I can't figure this thing out for the life of me, unless anyone has any further input I think I may pull the head off and check out the head gasket and valves, probably new chain tensioner an guides as well.

One last question, is it possible that the reference sensor could cause an issue like this? Not necessarily the sensor itself but perhaps the reference pin? I used a prybar to turn the flexplate when taking the bolts of for the torque converter. I don't remember seeing the pin or messing with it, I never took the flex plate off the engine either.
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Re: Rough idle/ misfire on an 85

Post by Ralph in Socal »

The head is not your problem and will be a waste of time and effort. Do you have an ecu to try out? I do have a spare 059 that I need to test for a couple of days before sending out. PM particulars and we can discuss specifics off line.

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Re: Rough idle/ misfire on an 85

Post by 86_6series »

+1 on what Ralph says, compression tests show no problem
with head or gasket.
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 »

Haven't updated in a while, this car is falling to the wayside as I've been focused on my other projects recently. I got a new ECU a while back and tried it out. No luck. Still the exact same running conditions. The only thing I haven't replaced is the AFM and it tested fine. I find it hard to believe the issue is sensor related at this point, seems like something mechanical. The guide on the slack side of the timing chain seemed loose and the chain seemed more lose than it should be, but not on the tension side. Still not sure what to do with this thing, everyone's continued input is much appreciated.
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Re: Rough idle/ misfire on an 85

Post by TimU »

Hi Teknoe46, did you have any recent updates about your issue please?
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Re: Rough idle/ misfire on an 85

Post by Pod »

TimU wrote: Wed Apr 13, 2022 12:20 am Hi Teknoe46, did you have any recent updates about your issue please?
Just re-read the thread and would like to hear the outcome as well!

I'm just wondering if the fuel FLOW (not pressure) is maintained when the engine is running? I've had problems with partially blocked fuel lines in the past as I have often mentioned............
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Re: Rough idle/ misfire on an 85

Post by KINGJOHN808 »

Following. I have a similar problem, awaiting new CPS and Speed Reference Sensor. While I’m in there, changing the FPR too. I have fuel coming through the rail and from FPR when I disconnected the hose, just no spark.
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Re: Rough idle/ misfire on an 85

Post by TimU »

Aloha John,

Are you getting spark from the coil so that you can identify which side of the spark-chain to start troubleshooting?

Checks:
- Loose engine earth straps (plural) - Commonly the one at the top-rear of the valve cover can get loose.
- Loose Battery terminals.
- Worn distributor cap & rotor button. Could also be gunked-up from an oil leak behind the rotor boss.
- Main Relay - One pin 87 can go faulty.
- Crankshaft Position Sensor - A faulty sensor will causes a loss of spark.
- Flywheel Speed Sensor - A faulty sensor will allow the car to start, but it will die soon after.
- Coolant Temp Sensor (blue head) - A faulty/disconnected CTS will stop the engine (unsure about spark).
- OBC Relay (under the dash) - A faulty OBC relay can cut power to the main relay.
- Cracked solder joint in the ECU

If you haven't already seen it, this article is an excellent troubleshooting guide:
https://hpsimotorsports.com/blogs/tech- ... ch-article
(PDF attached for posterity)

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