Hi,

I have a rather odd problem with my '88 420SEL. The car runs fine when cold, up untill the tempurature reaches about 50 degrees Celcius. Then it abruptly slows down starting at about 1500 rpm, then accelerates . This "slowdown and go" repeates untill I increase the rpm above 2500 . As if the brakes engage and disengage. Once the temperature reaches above 80, everything is fine again.

- Fuel filter new
- Added Sea Foam to gas tank few weeks ago.
- Engine temp sensors test ok.
- Distributor and cap are new.
- Air filter new.
- Spark Plugs new
- Added Sea Foam in the tank few weeks ago.
- I have also Seafoamed the engine through Intake Manifold few weeks ago.

Can defective Thermo Vacuum valve cause this kind of problems? I have not tested them yet.

Thanks for any help.

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Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)

It could be caused by lots of things but I would suspect a lean fuel mixture caused by excessive exhaust gas recirculation. Try disconnecting and plugging the vacuum hose to the EGR valve on the driver's side exhaust manifold. See if that changes your engine's performance. Mark

yes that egr being old might be leaking vacuum.I'd block it off the hose and try it.

__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran

Thanks guys. Will do.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)

Report back with an update after you try it

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1990 190E 3.0L

I disconnected the rubber hose off EGR valve and plugged it with a small screw. Unfortunately the problem continues. I know that the thermo valve between EGR valve and the throttle body opens at 50 degrees Celsius so that the vacuum can open EGR valve. That's the exact temperature at which hesitation starts and the engine bogs down until it reaches 80 degrees. I don't know of any other temperature sensor that opens at 50 degrees. If the problem was fuel related (fuel starvation) , I guess the engine would stall, right?

Could the problem be related to the catalytic converter?
Is this engine supposed to have warm up regulator (WUR)? I could not find it.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)

zorin:

On your question about a WUR : No, all mixture compensation is done in the ECU. Which brings us to,

Both the ECU and the ignition control unit (EZL) are supplied with coolant temperature information from a dual thermistor (one circuit for the ECU, one for the EZL). Both circuits have the same values and response curves. The unit is located just aft of the ignition distributor; when the plug is disconnected four pins are exposed. Try disconnecting and driving; when disconnected the infinite resistance signals replicate a permanent cold engine condition. If the problem ceases, you will then have to sort out which controller, ECU or EZL, is the culprit.

Thanks Frank. I will try that.
I noticed that this happens only when the car has been parked for long time e.g. over night. If it has been parked for only few hours, there is no problem when I start driving.
Thanks again.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)