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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53898
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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Marty! 1991 300 SL -- Good cold start and driving. Hot

Customer Question

Hi Marty! 1991 300 SL -- Good cold start and driving. Hot restart after being parked a short time results in difficult starting and then poor acceleration (several cylinders dropping out) for an extended period of driving. I first suspected rapid fuel pressure leakdown while parked but pressures are approx 90 psi running and holding at over 40 psi for 45 min when engine off. This test was with a nearly cold engine, though. I have K-Jet experience from the 70's and 80's on Audi & BMW, but am not familiar with many of the details of the system on this MBZ engine. Oh, it has fresh BERU wires and Bosch copper plugs with some miles which look good (none sooty or oily). Compression is good in two cylinders and not so great in the other four. It runs acceptably to my friends (the original owners) most of the time, but when it's running poorly it can be dangerous to turn left with oncoming traffic, for example. What are your thoughts? Thank you, Randy PS: The valve that circulates fuel to the low-side A/C line seems to be inop, but it hasn't been hot here lately (70's). The problem happens even when it's driven a relatively short distance (4 or 5 miles) in 60 to 70 deg weather. Thanks!
JA: How many miles are on the vehicle? And when was it last serviced?
Customer: I think it has around 150k. New cap wires and plugs about 5k miles ago.
JA: Are you fixing your 300 yourself? What have you tried so far?
Customer: Did you see my first message regarding what I have done? I am working on this for a neighbor/friend. I have scope, cis fuel gauges, etc and considerable experience with Bosch K-Jet systems on Audi and BMW. I'm looking for some specific thoughts and specs for this engine. Thank you!
JA: Anything else you want the mechanic to know before I connect you?
Customer: No, thanks
Submitted: 11 months ago.
Category: Mercedes
Customer: replied 11 months ago.
Additional info: I found that one of the 10A fuses in the OVP relay had blown. (It was REALLY blown, not just cracked.) I replaced the fuse and it has not blown since replacement (several 5 mi test drives). Replacing the fuse did not seem to affect the problem symptoms.
Expert:  Marty replied 11 months ago.

Hi Randy, sorry for the wait I have been away. I'd recheck the fuel system rest pressure when hot. I'd leave the gauge connected and see what your pressures are when you are actually experiencing the problem. It does sound like you are losing rest pressure somewhere and causing a vapor lock. If the problem eventually clears and it runs fine I'd say it is vapor locking. Dribbling injectors or a leaking pressure regulator in the fuel distributor or a bad accumulator or fuel pump check valve are some possibilities. If the only problem that you are experiencing with the car is the hot restart I would be looking for pressure loss.

Let me know if you have any questions.