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Old 02-02-2003, 03:38 PM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
Fuel starvation or transmission flaring?

Hi everybody,
when I took my '87 420 today for lunch, I experienced the following:
1. Accelerating from a stoplight "briskly" in second, all is well until suddenly the car "jerks" 3 times as if it shifts up into 3rd but doesn't. I guess I took the foot from the gas and it shifts into 3rd.
2. I do it again a few stops later with an eye on the rpms and sure enough at 3500 rpm it jerks again.
3. Driving with less gas and letting the transmission shift at 2000 rpms or so everything is fine.

Now first I thought this is the "flaring" but the engine is not really revving up. Then I thought it felt like the car is not getting enough gas. After all, I installed the fuel pumps only 2 days ago and there might be a kink in one of those fuel lines.

So at home I put it in Neutral and rev the engine expecting it to cut out at 3,500 but it does not. It seems to rev easily above 4000. I didn't rev it much higher.

So what do you think,
fuel starvation or transmission problem?

Could a resticted flow cut the engine out like that at 3500 rpms while driving (i.e.over a longer period) but then rev in Neutral without problems?

Should I just rev it up to 6000 in Neutral for a longer time to really see? I thought you are not to rev an engine without load that much.

Your thoughts are much appreciated.
Reinhard Kreutzer
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Old 02-02-2003, 05:47 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
It doesn't sound like a trans problem. it sounds more like a misfire condition. This could be fuel or spark related and appears to be load related.

Separating the two is easy with some tooling. If its a fuel problem then it is probably due to the pressure dropping. If its any kind of supply problem this is how it screws with the running. The CIS system requires constant fuel pressure.

A lot can be told by looking at (and replacing) the plugs.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 02-02-2003, 06:30 PM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
Thanks, stevebfl

I looked at it again and the hose from the fuel filter going to the front was routed incorrectly. Also, there is a pair of linked plastic clips that holds the fuel line to the accumulator together with the delivery fuel line and I guess prevents kinking. That plastic broke. I'll get one tomorrow. I also moved all the pumps and filter a bit more to the outside to give that little line more room to prevent kinking.

Taking pictures helped to see the "before" now, but I should have compared that before tightening everything on Friday.

I'll take it for a ride now.

Thanks again and keep your fingers crossed.

Reinhard Kreutzer
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Old 02-02-2003, 07:10 PM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
the car runs fine now. I can accelerate to 5000 rpms with no problem.

Shows how important it is to keep everything the way it is supposed to .....

Reinhard Kreutzer
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