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Old 12-14-2003, 11:23 AM
Posts: n/a
Thumbs up I have info on symptoms/diagnosis of a failing/failed Fuel Pressure Regulator!

I’m posting this information for the benefit of those interested in the symptoms of a failing/failed fuel pressure regulator.

My car exhibited these symptoms :

Bad fuel mileage (40% lower than usual).
Black smoke from exhaust.
When cold, car drives well and smokes lightly when accelerating.
When warm, car smokes lightly at idle, idle speed drops (so does oil pressure) and is lumpy.
Hesitates badly on acceleration (flooding) and clouds of black smoke from exhaust when car finally begins accelerating.
Driving with the EHA on or off is no different because it is a mechanical problem and the mixture is rich beyond compensation of the system.

I had the opportunity of seeing 2 faulty FPRs in my car.

My existing FPR was failing and exhibited the above-mentioned symptoms.I sourced a replacement from a wrecker which I found was actually a failed one and the car behaved even worse than when the existing one was installed.

Symptoms of existing failing FPR :

Symptoms as above.However, it was not venting fuel from the hose to the valve cover breather hose.

Car was barely driveable due to the severe hesitation in acceleration.

My 1st replacement was a used FPR sourced from a wrecker (which turned out to be a failed unit).

Symptoms of failed FPR :

Similar as failing FPR + severely surging idle.

Upon inspecting the FPR with the engine running, I clearly saw fuel flowing in the vent hose to the valve cover breather hose in rhythm to the surging of the engine.With this FPR, the car was totally undriveable (stalls when put in D).

Swapping the FPR with a known good unit resolved all issues.

Having read quite a number of posts on FPRs, I noticed that most mentioned that “if the FPR vents fuel, it has failed”.

My observation is that if it vents fuel, IT WILL BE NOTICED cause you will see a constant/surging flow of fuel thru’ the vent hose and the car will surge badly due to the venting of the fuel to the valve cover breather hose which goes to the intake.If you see a little fuel in the FPR behind the nipple of the vent hose and your car does not exhibit any of the above symptoms, your FPR is still OK.Even on my MB CD, it says that a little fuel here is permissible.

I did not have a fuel pressure gauge but I did notice a distinct difference between a failing/failed FPR and a good one.

On a failing/failed FPR, after turning off the engine, I disconnected the return line from the FPR to the fuel tank and VERY LITTLE fuel leaked from this line – almost negligible.So when the engine is running, fuel is sent to the FD possibly at 2 – 3 times the required pressure with almost no fuel being returned to the fuel tank.

On the good FPR, disconnecting the line resulted in a good amount of fuel leaking from the same line to the fuel tank.

This seems to me like an acceptable test to me to diagnose a failing/failed FPR.

The BOSCH part number for this part in my car is 0-438-161-001.

If any of the above information I have provided above makes no technical sense as usual, feel free to ignore me!Hehe…

Hope this post proves beneficial to others with similar issues.

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