Erratic engine saga

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sideshowbob
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Erratic engine saga

Post by sideshowbob »

My car an '88 Highline 635CSi has developed a very annoying fault, when it is warmed up the engine will momentarily cut out and then restart, it manifests itself with fuel economy meter going to infinity and a dip in the rev. counter, after this blip everything is normal. it then does it again, sometimes in quick succession resulting in car slowly loosing drive and can even cause car to stop.

If after this sort of stall after a few minutes it will restart and everything is OK until it starts again.....very annoying.

When this first started I changed the TPS and crankshaft sensor and it seemed to improve but then after a few days and a small amount of usage it has remanifested itself.

I thought it might be that the replacement TPS was faulty but I replaced this when it was showing these fault symptoms and the replacement did nothing to improve the situation.

My garage say they have done a smoke test and could not find any vacuum leaks etc.

I am now at a loss as to what it could be ....idle control valve ? thermostat sensor or what ???

Ant help would be most appreciated.
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Ralph in Socal
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Re: Erratic engine saga

Post by Ralph in Socal »

Swap the fuel pump relay. Horn relay is the same so just swap the two instead of buying one until you know it’s bad. Relay overheats during use then will reset when cooled down.

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)
sideshowbob
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Location: Hampshire UK

Re: Erratic engine saga

Post by sideshowbob »

Hi Ralph, thanks for the info. This afternoon I swapped the fuel pump relay for the horn relay. Took car for an extended drive and the fault did not appear for about 6 miles, in fact engine pulled strongly and smoothly and just as I was thinking I had solved the fault it re occured.

However and this might seem strange it was not so extreme, it just feels like a momentary miss, revcounter hardly moves but fuel indicatatot does a quick flick to infinity and then everything returns to normal. Also the frequency and severity never reaches a point when you think you are going to stall through lack of power.

Could this be related to the AFM in any way ?

Your help and knowledge is a great support !!

Robert
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Ralph in Socal
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Re: Erratic engine saga

Post by Ralph in Socal »

Likely a faulty main fuel pump but it could be the in-tank unit. One or both are causing the relay to overheat. The old fuel pump relay was compromised so it takes longer to reset. The replacement will soon start acting the same way if the true cause is not addressed. A clogged filter will also cause the same symptoms

Something is causing the relay to run hotter and it’s usually the main pump as that pulls much higher amperage than the lift pump. 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)
81 633 csi (retired)
sideshowbob
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Location: Hampshire UK

Re: Erratic engine saga

Post by sideshowbob »

Once again thanks Ralph. Plan is to source a new relay and the easiest and cheapest approach is to replace the filter and fuel pump, hopefully this will solve problem. If not will then replace in tank lifter pump. This is not so easy for two reasons as you have to replace pump with a complete unit which in UK is expensive and at present seems unobtainable.

Is it possible to take this assembled apart and just replace the pump ?
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86_6series
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Re: Erratic engine saga

Post by 86_6series »

I just installed the in tank pump roughly following this tutorial.

https://www.mye28.com/viewtopic.php?t=101410

But, only after I confirmed that the pump did not work.

Before you change the fuel filter and external pump, you should

test the fuel pressure to confirm a pressure problem.

Changing parts without confirming a problem will never lead to an

inexpensive solution.
86-635CSi
96-MB C220 SOLD
98-740i
71-Continental MK3
sideshowbob
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Location: Hampshire UK

Re: Erratic engine saga

Post by sideshowbob »

I do not think it is a fuel pressure problem, as Ralph suggests one of the pumps is perhaps faulty and while it can produce enough pressurised fuel it is drawing too much current causing the relay to overheat and cut out which causes engine to stall. When the relay cools down it resets and engine starts up again.

When the relay is operational the engine runs perfectly, this would not be the case if the pump was producing incorrectly pressurised fuel, I imagine it would run poorly if at all !!
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86_6series
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Re: Erratic engine saga

Post by 86_6series »

Very good point, so test the draw on the pump to see if is in fact drawing to many amps.
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Ralph in Socal
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Re: Erratic engine saga

Post by Ralph in Socal »

I would perform the filter replacement first and also verify the in-tank lift pump is working. A dead pump will also pull unnecessary amperage.

If you are replacing the main pump, you should also replace all the associated fuel lines. Use only high pressure fuel injection hose where necessary. 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)
81 633 csi (retired)
sideshowbob
Posts: 247
Joined: Mon Aug 01, 2011 2:18 pm
Location: Hampshire UK

Re: Erratic engine saga

Post by sideshowbob »

Thanks for that Ralph, car is going into garage next Tuesday, they are a BMW specialist and I am taking out their oldest technician who has experience on the 'older' BMW's.

Will discuss with him the relay and fuel pump situation and suggest like you a filter swap and then check and if necessary replace any faulty pumps.

Will let you know how I get on.....fingers crossed !!
sideshowbob
Posts: 247
Joined: Mon Aug 01, 2011 2:18 pm
Location: Hampshire UK

Re: Erratic engine saga

Post by sideshowbob »

Quick recap....my '88 635CSi engine kept cutting out and then restarting, it did this after about 9 miles or when warmed up. The cutting out was erratic with a drop in revs and the fuel econometer swung violently to the left, it was if you had violently taken your foot off the throttle.
It might do it once and then recover or it could do it several times until you basically stalled. Car would them immediately restart.

It was suggested I should replace the crankshaft sensor, this was done with a relatively cheap after market version, problem persisted. It was then suggested a better quality crankshaft sensor would cure the problem, a Bosch version was sourced after several weeks of searching...this did not solve the problem.

Replaced the throttle position sensor with a second hand one...again this did not solve the problem.

Fuel pump relay was then suggested as causing the cutting out and the relay ? pump could be faulty. Pumps were wired bypassing the relay and still the problem persisted.

Finally had the ECU removed and sent to ATC Drivetrain (01543 427900) and they tested the ECU which had developed a fault, they then supplied one of their reconditioned units coded similarly to my failed unit.....problem solved, cost £145.00

Thanks to everyone on here who have suggested possible faults and remedies but I was so relieved to get the phone call from my garage this afternoon to say the problem was cured , and a big thanks to ATC for all their help.
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Ralph in Socal
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Re: Erratic engine saga

Post by Ralph in Socal »

Congratulations on getting it fixed at a relatively affordable cost. Please provide the last 3 digits on the ecu again (150 or 179) for future reference. Thanks

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)
sideshowbob
Posts: 247
Joined: Mon Aug 01, 2011 2:18 pm
Location: Hampshire UK

Re: Erratic engine saga

Post by sideshowbob »

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86_6series
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Re: Erratic engine saga

Post by 86_6series »

Thanks for letting us know how the problem was solved.
86-635CSi
96-MB C220 SOLD
98-740i
71-Continental MK3
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