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Rough idle/ misfire on an 85

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Teknoe46
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Rough idle/ misfire on an 85

Post by Teknoe46 » Wed Jul 21, 2021 2:32 am

Hi all, first post here. Just purchased a new to me E24, an auto 1985 635.
I bought it knowing that it would need some work, but wasn't expecting it to be so much of a hassle. Wouldn't go into drive or run well when I first got it.
Since purchasing it I've replaced the forward clutch packs in the auto trans ( that fixed the trans issue ), the distributor cap and rotor, spark plugs, plug wires, fuel pressure regulator, as well as the main and fuel pump relays.
The car currently has a very long crank before it starts up. Once it starts it takes some throttle to keep it alive until it warms up some. Once it can stay running it idles really rough, and feels like it's misfiring. Runs even rougher once you put it in drive. Though it revs up freely and feels smoother in the higher rpms.
So far I've gone through and tried to be as thorough as possible in diagnosing the issue. Compression at almost exactly 150 psi across all 6 cylinders, both fuel pumps are coming on. Cleaned and tested the ICV, checked out good. Cleaned and tested the AFM, values were good on that as well. Tested resistance on the coolant temp sensor and it was good. All 6 cylinders are getting spark. I have plugged off the CSV to eliminate it from the equation, as I live in South Texas anyway. Removed the fuel rail and jumped the fuel pumps on to verify the injectors aren't leaking. Vacuum lines look good, smoke tested and couldn't find any air leaks, engine dies if you removed the dipstick. I've also noticed that idle doesn't change much if you remove a plug wire while the engine is running.

Sorry for the paragraphs but wanted to be as in-depth as possible, looking forward to any input you guys may have in diagnosing the issue. The only thing I haven't been able to verify for sure is the fuel pressure, although I know both pumps are working.
Thanks for your time and help!

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Re: Rough idle/ misfire on an 85

Post by Pod » Wed Jul 21, 2021 7:55 am

You could try substituting a "known good" ECU and see if that makes any difference?

Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Wed Jul 21, 2021 3:57 pm

Do you know of any pinouts or diagrams for testing the ecu? I don't have a spare one on hand and would like to verify the issue before throwing parts at it especially given the price of the ECM. The car has the purple label motronic unit for what it's worth

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Re: Rough idle/ misfire on an 85

Post by hornhospital » Wed Jul 21, 2021 4:02 pm

This is a link to the BMW Electronic Troubleshooting Manual for your 85 635CSi:

http://wedophones.com/Manuals/BMW/1985% ... Manual.pdf

That will have a pinout of the ECU.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"; '95 318is "Bebe"

Pod
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Re: Rough idle/ misfire on an 85

Post by Pod » Wed Jul 21, 2021 4:22 pm

Teknoe46 wrote:
Wed Jul 21, 2021 3:57 pm
Do you know of any pinouts or diagrams for testing the ecu? I don't have a spare one on hand and would like to verify the issue before throwing parts at it especially given the price of the ECM. The car has the purple label motronic unit for what it's worth
If you buy one second-hand and it makes no difference, you could keep it as a spare, or resell it at a later date. Won't cost much and will save a whole load of time checking pins.

Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Wed Jul 21, 2021 11:14 pm

I should add, today pulled the ECM apart and didn't find any clear signs of failure. No burns or burnt smells, a few small corrosion spots but nothing major, brushed and used contact cleaner to clean it off. Also reflowed alot of the connections on both boards. Felt like it ran a bit better upon startup but overall ran pretty much the same.

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Re: Rough idle/ misfire on an 85

Post by miklilmag » Fri Jul 23, 2021 3:11 am

Hello,
reading thru your list,
You are well on your way,
Everything needs to be done anyway, soon or later!

I do not see a fuel filter replacement,

Clogged catalytic converter?

With a fresh/ fully charged battery (13.8V+) does it still crank a while before firing.

Franke

Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Fri Jul 23, 2021 2:03 pm

I'm hesitant to mark it as a fuel pressure or clogged cat given the fact that it revs up so freely. Doesn't bog down or try to die when you rev it. And yes, I leave the car on a trickle charger and I'm seeing about 13.5V before cranking. Just rebuilt the alternator, changed or cleaned all the ground wires and replaced the battery as well.
The fuel filter and both pumps were done about 8 months ago.

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

Post by Ralph in Socal » Sat Jul 24, 2021 3:33 am

Long crank on cold start indicates the cold start system is not working properly. Rough idle yet smooth high rpm running indicates a rich mixture

Verify the cold start injector is working. Nothing like pulling it off and watching fuel spray!! A noid light or checking for voltage will only verify electrical supply.

There are 2 (sometimes 3) Two-wire sensors on the thermostat housing. The Thermotime Switch (triggers the cold start system) and the Coolant Temperature Sensor are right next to each other and it would be easy to mixup the wires. Verify that the wiring is correct. Good luck

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
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Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Sun Jul 25, 2021 2:33 am

Mr. Ralph thanks for the input. It certainly smells like it's running rich. As far as the cold start valve is concerned, I have deleted it by plugging it off at the valve itself as well as at the fuel rail. So I'd say it can safely be ruled out. Could you help point me toward what, other than the coolant temp sensors controls the fuel regulation?

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

Post by Ralph in Socal » Sun Jul 25, 2021 4:41 am

Oxygen sensor inputs to the ecu adjusts a/f mixture so make sure that’s working properly. Output should fluctuate between +1V to -1V from what I have read

Ralph

Edit - there are also several versions of the coolant temp sensor so make sure you have the correct one. The throttle position sensor is also critical. It requires proper adjustment and function to put the ecu in idle control mode

Unplugging the idle control valve while it’s running should increase idle rpm. If not the system is not functioning properly
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)

emac
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Re: Rough idle/ misfire on an 85

Post by emac » Tue Jul 27, 2021 1:29 am

I had a bmw that started running rough, very rich, hard to start and would run best under acceleration. It turned out to be a bad fuel pressure regulator on the fuel rail. May be worth checking out.

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

Post by Ralph in Socal » Tue Jul 27, 2021 4:51 am

emac wrote:
Tue Jul 27, 2021 1:29 am
I had a bmw that started running rough, very rich, hard to start and would run best under acceleration. It turned out to be a bad fuel pressure regulator on the fuel rail. May be worth checking out.
Agreed on the FPR. The cold start valve fuel supply nipple that is not currently used is a perfect place to attach a pressure gauge

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)

Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Tue Jul 27, 2021 2:39 pm

I just installed a brand new fuel pressure regulator on the car and there was no observable difference when it was installed.
I'll be pulling the upper timing cover later today to verify the timing is correct and hasn't jumped a few teeth.
I tried to drive the car over the weekend thinking it may just need a good old "italian tune-up". Barely made it around the block, the car surges, makes normal power and totally bogs down in short bursts.
If anybody has a known good spare purple label motronic ECM for sale please drop me a line.

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Re: Rough idle/ misfire on an 85

Post by hornhospital » Tue Jul 27, 2021 9:10 pm

If the O2 sensor has failed or lost connection the ECU will default to full rich. Could be the heavy fuel smell.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"; '95 318is "Bebe"

Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Tue Jul 27, 2021 11:04 pm

Also to add, I finally bought the connector I've been missing for my fuel pressure gauge. Car make 40psi when I jump the pumps on, left it on for about 20 minutes and still has just barely dropped to about 30 psi. Haven't checked it while running because I have the valve cover off at the moment.

Also is it a known issue for m30's to jump timing? Chain is tight around the cam sprocket but the chain and guide itself on the non-tensioning side is looser than I would expect

Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Wed Jul 28, 2021 1:52 am

More input from tonight- Verified the timing is correct, re-adjusted the valves to .09 on the cam side. Adjusted on the valve side the first time and come to find out some were a bit out of spec. Fired it up and it ran the same, slightly worse if anything. Read the fuel pressure while idling and it hovered right around 35 pretty steady.

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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Wed Jul 28, 2021 3:34 pm

Just ordered a new o2 sensor, coil, and TPS. Will update once they come in.

Going off what Ralph mentioned on the ICV, it's about a year old and I just tested it for resistance and cleaned it. However I don't see an idle increase when I unplug it while the engine is idling.

Another point I forgot to mention, when I attempted to drive the car the other day, I did get what sounded like a backfire out the exhaust when the car bogged down and I tried to give it throttle. That was one of the things that was leading me to believe it was a timing/ valve issue. I've yet to do a leakdown test as I don't have an air compressor at the house. Holding out hope that it isn't burnt or cracked valves.

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

Post by Ralph in Socal » Wed Jul 28, 2021 7:18 pm

Don’t worry about a leak down test. If compression is great leak down won’t show anything

The throttle position sensor can be tested. It is a simple on/off switch both at idle and at WOT. Simple continuity metering will prove good or bad. Once installed it has to be adjusted so that the idle circuit is on at idle then off as soon as the throttle is moved. It could be the reason the ecu is not going to idle control

Great testing procedure on YouTube. Search “DIY BMW Throttle Position Sensor”

Ralph

Edit - backfire could just be excess gas burning in the exhaust
Last edited by Ralph in Socal on Wed Jul 28, 2021 7:30 pm, edited 1 time in total.
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)

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

Post by Ralph in Socal » Wed Jul 28, 2021 7:23 pm

hornhospital wrote:
Tue Jul 27, 2021 9:10 pm
If the O2 sensor has failed or lost connection the ECU will default to full rich. Could be the heavy fuel smell.
BMW made the ecu default to full rich if a sensor has failed to provide proper input. Smart since the car just runs poorly instead of burning up the motor
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)

Teknoe46
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Joined: Fri Jul 16, 2021 3:55 am
Location: TX

Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Thu Jul 29, 2021 2:02 pm

Ralph, thanks again for your continued input.
After checking out the video, the TPS is indeed out of spec, I can't activate the idle switch when having it fully turned within the adjustment slots.
I removed the screws and turned until I could hear the click while the car was idling. The idle has a noticeable change when I do this, but overall the issue persisted unfortunately. But at least I have a new one on the way and can 100% rule it out once I get it on.
Also noticed that when I hold the TPS to active the idle switch I do get an idle increase when I unplug the ICV. So I would imagine the ICV requires the idle input from the TPS to function, which it has not been getting.

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

Post by Ralph in Socal » Thu Jul 29, 2021 5:38 pm

Make sure the throttle body is clean. Typically gets buildup that prevents proper air bypass at idle. The butterfly also has to be adjusted properly.

Making progress!!

Ralph
There is a very fine line between "Hobby" and Mental Illness.

85 635csi Zinno Auto
84 528i Euro
83 635 Euro Arktik
81 528i Kastanien 5-speed
88 528e Bronzit (Granpa Car)
86 535i Auto (For Sale)
81 633 csi (retired)

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Re: Rough idle/ misfire on an 85

Post by GRNSHRK » Thu Jul 29, 2021 6:25 pm

Not so sure about this statement, Ralph :-?
Don’t worry about a leak down test. If compression is great leak down won’t show anything
A compression test will only show one of the potential issues that a leakdown test will reveal :-"

My engine has decent compression numbers but poor leakdown results, showing a need for rings in a few cylinders :(

A true leakdown test will show not only if your mill is tight or not, but also where it''s not =D>

My engine runs fine but I suspect that the poor leakdown numbers were the result of using a POS K&N filter for years [-X

Sorry, but I don't have any advice regarding these rough idle/misfiring problems :-({|=
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Re: Rough idle/ misfire on an 85

Post by marc79euro645 » Thu Jul 29, 2021 11:25 pm

What do the new plugs look like now? Are they looking fouled?

Teknoe46
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Re: Rough idle/ misfire on an 85

Post by Teknoe46 » Tue Aug 03, 2021 3:32 am

So new parts came in today, Put the new TPS and O2 sensors on. No real difference, if anything I feel like I'm going backwards with this car. Seems like it ran better when I first got it. At this point the car will die as soon as it starts up if you don't continuously give it throttle. I'm about at my wits end and will probably just put an m52 in it if I can't sort it out in the next month or 2.

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