SC Fuel pump Check Valve - Can they be repaired/cleaned?

spiff · 09-07-2006, 08:43 AM

I'm trying to diagnose a warm start issue on my 80 SC. I'm getting the pressure tester soon to test, but in the even that it's the check valve at the fuel pump, is it possible to just try and clean it out or is the failure mode that the seal on the valve may no longer work?

Also, is there an easy way to tell if it's the check valve vs. the accumlator that's bad? The manual doesn't say.
Thanks in advance.

phiba · 09-07-2006, 09:58 AM

I don't think there is an easy way to determine if it is the accumulator or the check valve without checking each one. It is a lot easier to change the accumulator than the fuel pump, and less expensive too. So this is probably the best place to start, and you can change the fuel filter while you are at it.

It is said you can check the accumulator after removing it by blowing into the drain port on the bottom and seeing air leaks out the input/output ports on top. This would indicate a ripped diaphram. But I think the diaphram spring can fail also. In my case, the accumulator did not seem to have a leak, but still, changing it and the fuel filter appears to have solved my warm start problem.

'78 SC · '78 SC's Garage

I've also been chasing a hot start problem. I ran all the usual tests (which only confirmed the system doesn't hold pressure after shutting off the engine). After determing the expense of randomly swapping parts, I decided to try some more diagnostics...

I rigged up my CIS gauge with some adapters and a regulated air supply. I applied pressure from the regulator, then used the valve on the CIS tester to isolate the gauge and component being tested from the air supply. Here's what I found:

1) WUR - leaks down slowly to 1 bar, then steady. (maybe OK; the CIS leakdown test allows the pressure to drop to 1.1 bar after 20 minutes)

2) Fuel Distributor - leaks down to 2.5-3 bar, then steady. (per spec for the internal regulator)

3) Fuel supply (line leading from pump) - holds 3 bar (pump check valve probably OK)

4) Fuel Accumulator (connected to s-pipe that leads to filter) - drops immediately to < .5 bar. Per Bosch manual, should hold 2 bar. BINGO!

I've replaced the accumulator, but am in the process of reinstalling the engine. I'll let you know this weekend if that cured the hot start problem.

spiff · 09-12-2006, 09:46 AM

Just as a follow up, I didn't ever get a chance to test fuel pressures using the gauge. I was just going to try replacing the check valve (with the type that goes in the Fuel Distributor instead of the pump). Before I actually bought the part, I kept thinking that my hot start problem starting happening very suddenly after I did a major vacuum job on the interior. It got me to thinking that maybe the o2 sensor relay under the passenger seat might have been affected (wires/connection loose). This might have been enough to affect warm start where during cold start the added fuel covers up the bad running. Well, I tried to reseat the relay and Presto! the car starts right up again on warm start.

Would this make sense? Since then in the past week I've noticed one other time of many that the car has had trouble starting when warm, but after wiggling the connection, the problem has again disappeared.

The car runs fine, has great power, starts up fine when cold (although hunts for idle a bit). I'm thinking this is related to the o2 sensor wire being broken and the idle mixture being just a little off.

Any thoughts?

I just want to see if I'm imagining something or should I just go ahead and replace the $20 check valve?

ruf-porsche · ruf-porsche's Garage

Not enough info to give a good opinion as to what you should do.

Hookup the gauge and give us the numbers for system pressure, regulated pressure cold, regulated pressure warm and residual pressure with the WUR isolated from the system.

Oracle · 09-12-2006, 09:22 PM

Its possible one or more injectors is leaking hence the loss of pressure, my mechanic with his 15+ years experience as well as his colleague with 14+ years once told me they have never seen an accumulator failed...

JK-81SC · 09-13-2006, 03:28 PM

If your accumulator has a fitting on the bottom, then it's easy to check. From what I've read here, the bottom fitting is for the return of fuel that leaks past the diaphragm. If the diaphragm is good, there should be no fuel coming out of the accumulator when the bottom fitting is removed. Is this true?

The accumulator looks like this: