GJB928

So I took my '78 euro 5 speed out to get the windows tinted the other day. On the way to the shop I experienced a pretty significant stumble once it warmed up. At one point it bucked and stalled. I coasted to a stop and tried starting it and it fired right up. I made it to the shop. I picked it up that same evening and it started right up, idled fine and drove ok until it heated up. About 7 miles. Then it started acting up again. Bucking really hard and would not pull over 4k rpm at full throttle.

Today I did a CIS pressure test and these are my results.

WSM System Pressure specs = 75 - 85 PSI
Test = 75 PSI steady

WSM Cold Control Pressure specs @ 76º Ambient = 24 - 33 PSI
Test with connector disconnected = 20 PSI

WSM Warm Control Pressure Specs = 40 - 46 PSI
Test = 42 PSI Slowly climbed from 20 to 42 when I plugged in the connector to WUR

WSM Leakdown Specs =
@ 10 min 29 PSI
@ 20 Min 24 PSI
@ 30 Min 23 PSI

Test=
@ 10 Min 39 PSI
@ 20 Min 37 PSI
@ 30 Min 36 PSI

So other than The Cold Control Pressure being low (no cold start problems ever) the other #s are within spec.

The only test I haven't done is the flow test. Still trying to figure out how to do that one since my test rig doesn't have a pressure bypass valve.

Can I just disconnect the test hose from the WUR and flow that into a beaker?

Any suggestions on what I should look for?
Am I missing something in these test #s?

Rich9928p

I suggest dropping a note to Larry at CIS Flowtech, he knows these systems well.

EMAIL: cisflowtech at gmail.com (use the @ symbol).

Rich
928 OC Membership Chairman
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue Cover Girl too!
1979 928 Euro 5-liter track beast

drooman

The flow/ volume test really should be done from the return side of the fuel dist..which is a banjo fitting. You can also do it from the hose at the top of the gas tank at the fuel sender..it's easier to rig up if you don't have a banjo fitting test hose in your tool stash.

If it fails the flow test, check for low voltage to pump, change filter.

If it passes the flow test, leave the guage on and rig it up so you can see it from the driver seat..I tape it to the windshield. Drive around until it acts up, if the pressures are dropping you could have a failing pump or pump relay or pump power supply issue.

If it acts up and pressures stay steady..start thinking ignition.

kiwiokie

May not be the cause but while you are messing with the WUR there is a fine metal gauze filter on the inlet side that typically gets plugged with junk that should be cleaned out.

GJB928

Drooman, fuel hose at the top of gas tank is an excellent idea. I'm going to do that today. Thank you.

GJB928

Makes sense to do that while the fitting is out. Can't hurt right?

GJB928

I did the flow test today and it only produced 770cc of fuel. So I ordered a new fuel filter and will check voltage to pump tomorrow. Looks like I've narrowed down the problem.

drooman

If that filter is original or been on there for many years prepare for a fight. If the fittings are crusty fluid film them asap and give them a week to soak.

Before you do the filter, jump the pump power AND ground right from the battery and try the flow test. You'll often find the pump runs faster, louder, and pushes a lot more fuel with a full 12v.

Be extra careful with jumper wires and sparks back there if doing the flow test at the top of the tank.

GJB928

I did the flow test and got a total of approximately 700cc in 30 seconds. So I replaced the fuel filter and retested. Flow increased to 880 cc. Since it was below the spec of 1150 cc for my car I didn't bother test driving it. I ordered and installed a new Bosch fuel pump. Tested flow again and it was less than the 1st test. Said what the hell and took it for a test drive and problem solved.

I'm happy that it is back to running correctly, but I hate replacing parts chasing a problem. Granted the fuel filter definitely needed to be replace since it appeared to be the original Purolator that's 40 years old. But I expected the pump to show more flow.

Is it possible that at the different times I performed the fuel test, the temperature of the WUR could of had an effect? At all times I performed the flow test the engine was cold, however sometimes I tested in the morning when temps were in the 60s and other times in late afternoon when it was low 100s..

Also flow test was done from fuel return hose to the tank.

Rich9928p

Does your 1978 928 have an in-tank fuel pump? You can tell if there are wires connected to terminals where it would screw into the fuel tank. My 1979 928 did have both in-tank and external fuel pump.

If it has an in-tank pump that isn't working, that will cause a low fuel pump flow rate.

If it doesn't have an in-tank fuel pump, there may be a different pump part number for an external pump that wasn't designed for the in-tank pump .... Alex, do you know?

If there isn't an in-tank pump, the strainer could be clogged up or missing and an unfiltered chunk of crud could be restricting the intake.

After sitting for more than a decade the fuel pressure accumulator in my 1979 started to leak. The fuel pressure accumulator could also be a source of flow restriction.

Rich
928 OC Membership Chairman
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue cover girl too
1979 928 Euro 5-liter track beast

GJB928

No in tank pump. I just cleaned fuel tank and inspected strainer last summer when I replaced fuel tank outlet seal. Leak down numbers from fuel pressure test indicates accumulator is okay.

Maybe flow restriction on return to tank? Although wouldn't that raise system pressure?

Puzzling.

But good news is that it's running great!