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Weird engine surge on overrun

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Location: Christchurch

Weird engine surge on overrun

Post by Rangi246 » Mon Jan 22, 2018 7:37 am

Hi all

I'm hoping someone can help me with a problem our car has developed. Its an 86 635csi auto and for some reason it surges on overrun but only when the a/c is on.

When you lift your foot off the gas, the revs drop to about 1,000 and then they pop back up to about 1400-1500. When the a/c is off the car does this once which I understand is normal.

When the a/c is switched on though, the car will complete this cycle up to 6 or 7 times before settling down. The effect is at a slow cruising speed, the car is pulsing along the road. If you put your foot on the gas or brake, it settles down. Its slowly getting worse as it was doing it about 3 times max but now its up to 6 or 7.

When the car has stopped it idles perfectly at about 650/700 rpm.

So far I've replaced the AFM and apart from the car running a bit better with a new unit, it hasn't made any difference to the surging when the a/c is on. Maybe the ICV?

Any thoughts?


Posts: 9
Joined: Sun Mar 26, 2017 9:03 pm
Location: Christchurch

Re: Weird engine surge on overrun

Post by Rangi246 » Wed Jul 11, 2018 1:59 am

So an update on this issue. I noticed the car was only doing this when it had warmed up.

Replaced the following and the car was still doing it:
Idle Control Valve
Air Flow Meter
Coolant Temp Sensor
Tested the TPS and all ok
Replaced a few collapsed vacuum hoses
Eliminated a few small vacuum leaks
Tested the Fuel Pressure Regulator and almost everything else under the hood

No change :-x

Took the car to the garage and got them to put in on the gas analyzer. It showed the exhaust was WAY off spewing all sorts out the back.

Looked at the plugs and found the car was running lean.........and then one evening I noticed the ECU was flapping around inside the dash. I had a look and guess what, numb-nuts previous owner put in an 073 type for a 325i. I've since sourced an 061 type, installed it and problem solved. Idles perfectly, no lean plugs, no bouncing idle on deceleration, in short, its going really well! \:D/

Here's what I think the previous owner did to fix a crook ECU.

I suspect the original ECU bombed out so he replaced it with one from a 325i. The idle bouncing on deceleration started so instead of getting the correct ECU, he put in a smaller thermostat, jammed it open and put it in backwards. The effect being that the ECU told the car to run rich as it never got to normal operating temperature. Lean running and idle problem solved. And this dude was a mechanic :shock:

That was until I replaced the thermostat and fan clutch and the temperature gauge started reading correctly.........


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Re: Weird engine surge on overrun

Post by Brucey » Wed Jul 11, 2018 6:10 am

I'm amazed the car ran at all. Just shows you can't take anything for granted....


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Re: Weird engine surge on overrun

Post by hornhospital » Wed Jul 11, 2018 8:05 pm

Rangi246 wrote:
Wed Jul 11, 2018 1:59 am
And this dude was a mechanic :shock:
I would take issue with calling anyone that bolloxed up a job that badly a "mechanic". I'm glad you got it all sorted. Nice work! =D>
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"

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