1988 Hard Start, Stumble, Dies

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josh
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Location: Arkansas

1988 Hard Start, Stumble, Dies

Post by josh »

A coupld of months ago my '88 one day just wouldn't start right away, then stumbled as soon as any gas was given to it, then died when put into gear. I finally have some time to start trying to figure it out, and...well, I'm kind of stumped. It acts like it has a bad vacuum leak, but I can't find one and it smells rich (though that could just be that I haven't let it warm all the way up yet). I have taken apart and cleaned the intake and AFM. Traced all the vacuum lines looking for cracks, disconnects, as well as the the wired for the sensors. I also cleaned all the sensor connectors.

I'm going to test the fuel pressure later today or early tomorrow when I get the time. Does anyone have any suggestions?

Additional Info: If you gently apply the throttle, it will rev up, but it you go quickly, it will immediately die.

Thanks
1988 635Csi Auto
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Ralph in Socal
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Re: 1988 Hard Start, Stumble, Dies

Post by Ralph in Socal »

Check AFM outputs at the connector. There are different specs for the different AFMs

I disabled my fathers 528e by disconnecting the AFM. Car would start and idle well, RPMs would slowly rise by feathering the throttle pedal. But the engine would die if the pedal was pushed too hard. Good luck

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

85 635csi Zinno Auto
84 528i Euro
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86_6series
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Re: 1988 Hard Start, Stumble, Dies

Post by 86_6series »

It sounds like a vacuum leak to me also. Visual inspection may not be enough.
Do a smoke test to be sure.

Then follow this. It should give you direction to a solution.

https://www.hpsimotorsports.com/motronic-
86-635CSi
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josh
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Location: Arkansas

Re: 1988 Hard Start, Stumble, Dies

Post by josh »

It has been a very difficult year, and the car sat for a long time. I was able to go out from time to time to check bits and try to start it, but nothing ever worked. Today, I grabbed an extra DME that I had in the garage, and replaced it. Tried the car and it...started right up like nothing was wrong. I turned it back off, and tried it again just to be sure. It fired right up again. I backed it out into the driveway and let it idle for a few minutes. Everything seemed fine. I turned it back off, and decided to give it a quick wash and drive it around the block to see if it was real. Sprayed all the dust off, hopped back in, turned the key, and...nothing. Just spun the engine but never caught. So...now I'm thinking it is something electrical. Hopefully, I didn't just fry a DME somehow.
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josh
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Re: 1988 Hard Start, Stumble, Dies

Post by josh »

Should note: I just went out and tried it again after letting it sit for a couple of hours. It is back to running like it has a really bad vaccum leak or misfire. ie: Stumbles badly off idle, dies when put into gear.
The fuel pressure was fine. If I remove the oil fill cap on the valve cover, it stumbles more. If I disconnect the AFM it dies completely.

Could this be an issue with the Crank Position Sensor, or something like that? It's baffling. I wish I had more time to work on it, or get it to my shop, but I can't right now.
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Ralph in Socal
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Re: 1988 Hard Start, Stumble, Dies

Post by Ralph in Socal »

Your car should be Motronic 1.3 with the 154 or 179 ecu. My experience is limited with the B35 system apart from there is one crank position sensor up front. They do go bad from heat cycles. Just strange that it ran great for a while then back to crap. The sensor is not expensive so it’s an easy option to replace.

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)
josh
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Joined: Wed Oct 01, 2014 2:24 pm
Location: Arkansas

Re: 1988 Hard Start, Stumble, Dies

Post by josh »

Yeah, the two I have are 179. And I’m with you. The fact that it ran great for a bit today is really messing with me.

I don’t want to just throw parts at it, but, I guess I can grab as many small cheap (relatively) things that I can and see if anything makes a difference.

Going to get the fuel pressure gauge out again tomorrow if time permits. I can’t put my finger on why, but I have an urge to test the pressure regulator. Probably just grasping at straws, but it’s something I can do while I wait for parts to arrive.
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Lbs6
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Re: 1988 Hard Start, Stumble, Dies

Post by Lbs6 »

From where I'm sitting it looks more like an ignition problem than fuel. Most of my experience is with the early e12 cars but it shure sound like a weak spark. Intermittent connections to the coil primary side, bad or failing coil/wire. More issues after washing sounds like bad connections. There is a grommet for the computer harness that is usually compromised allowing water directly above the glove box to cause grief.

Good luck
Leonard
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josh
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Re: 1988 Hard Start, Stumble, Dies

Post by josh »

I did check to make sure that the computer was, in fact, dry. I know that there is a tendency for moisture to leak into that area when seals start to dry out.
Interesting about the weak ignition theory. I’ll get out there with my multimeter this afternoon and see if anything seems amiss.
1988 635Csi Auto
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josh
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Re: 1988 Hard Start, Stumble, Dies

Post by josh »

Fuel Pressure Regulator checks out fine. I have gone over everything with the multimeter and can't find anything out of place.
Started up again and drove fine, except for a stumble on acceleration at high speed, but that could just be bad gas from sitting for so many months (I hope). Still trying to figure this thing out.
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josh
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Re: 1988 Hard Start, Stumble, Dies

Post by josh »

My wife said she noticed some white smoke coming out of the tail pipe as I left the house. So, I checked the oil dipstick and, while it wasn’t milkly, there seemed to be some kind of reddish residue on the edges of the dipstick. So, I carefully drove it to my workspace and took the head gasket off to see if I could find any evidence of coolant. Nothing. Looks perfectly fine aside from needing an oil change.
Now that I have it somewhere I can actually get into it, I’m going to do a compression check on the cylinders, and go through the wiring and vacuum lines again. Anyone have any other suggestions?
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Da_Hose
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Re: 1988 Hard Start, Stumble, Dies

Post by Da_Hose »

One very sneaky leak is the oil dipstick. If you don't have a good seal at the stick, or down where it connects to the pan, you will get that rough running condition. Same thing could happen if your oil cap has a bad seal, or if the oil drain tube from head to block has bad seal.

Jose
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TimU
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Re: 1988 Hard Start, Stumble, Dies

Post by TimU »

86_6series wrote: Sat Aug 29, 2020 12:56 pm It sounds like a vacuum leak to me also. Visual inspection may not be enough.
Do a smoke test to be sure.

Then follow this. It should give you direction to a solution.

https://www.hpsimotorsports.com/motronic-
Just a follow-up - The URL has changed for the HPSI page.
Here is the new link:
https://hpsimotorsports.com/blogs/tech- ... ch-article

I've also created an archive of the page in the Wayback Machine for posterity.
Unfortunately some of the pictures are already archive links from 2016, so they don't load in the new page's archive for some reason.
http://web.archive.org/web/202204210437 ... ch-article

And for good measure, I've attached a PDF of the page content (with pics) for download and printing.
04/1986 635csi Euro spec
Australian Delivered, RHD
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