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TOPIC: 1978 928 5 speed fuel pump delivery test.

1978 928 5 speed fuel pump delivery test. 21 Apr 2010 08:29 #222


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I recently did the fuel pump delivery test on my '78 5 speed 928 at the return line on the system pressure regulator that is part of the fuel distributor as outlined in the Bosch Fuel Injection and Engine Management publication. Shop manual calls for at least 1360cc = 45.9 ounces after 30 seconds-2 fuel pumps. I collected just about 44 ounces, is this sufficient considering I used a 7" hose with a banjo fitting and connected another 53" hosetotal 60" that ran up from the fuel distributor over the fender and into a container. I disconnected the 53" segnent of hose and lost some in the process also there was a seep at the fuel feed line which I have since tightened.Used a $12 watch with second hand for time. All things considered does 44 ounces seem sufficient? Any knowlegeable response is greatly appreciated. Greg Heinrichs
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1978 928 5 speed fuel pump delivery test. 21 Apr 2010 08:43 #223


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I know from experience that much of the fuel delivered is just return to the tank. I would expect at wide open throttle this would be reduced, but with consideration to your numbers, I would speculate that you have sufficient fuel flow, with the possible exception of under wide open throttle when every drop of fuel might be required.

My knowledge on this one, is definitely just some experience in seeing how much fuel was returned to the just installed fuel cell in my '80 Euro S.

What is the problem you are chasing?
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1978 928 5 speed fuel pump delivery test. 21 Apr 2010 09:38 #224


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Thanks for the response. Car will sometimes start and idle correctly but stumbles, hesitates, misfires when driven. Then stalls and is hard to start. Then won't start, then might rev for a second and stall, then start and idle seemingly correctly only to start the cycle again. Checked cold start valve which was not spraying- replaced and the above problems surfaced again. I am troubleshooting now from square one.
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1978 928 5 speed fuel pump delivery test. 21 Apr 2010 09:58 #225


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A common problem is the electrical wire that runs from a control box on the passenger side firewall at about the front most point of the motor, the wire runs to the distributor across the front of the motor. If it is damaged, it can lead to many ignition issues, which your symptoms are among.

It is a special wire, and relatively expensive

Greg Nichol's site has some great tips, particularly for an older 928 in my opinion: http://nichols.nu/tips.htm
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1978 928 5 speed fuel pump delivery test. 21 Apr 2010 10:08 #226


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Familiar with green wire issues which shuts off fuel pumps. My pumps come on when I am attempting to start engine so I don't think it is green wire related but you never know. I may have to reconsider in the future. Just trying to definitively determine if my fuel delivery as described is sufficient for me to move on to control pressure tests.
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1978 928 5 speed fuel pump delivery test. 24 Apr 2010 20:41 #233


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vze2jshn;244 wrote:
Thanks for the response. Car will sometimes start and idle correctly but stumbles, hesitates, misfires when driven. Then stalls and is hard to start. Then won't start, then might rev for a second and stall, then start and idle seemingly correctly only to start the cycle again. Checked cold start valve which was not spraying- replaced and the above problems surfaced again. I am troubleshooting now from square one.

The fuel you accumulated in your test was certainly sufficient. Pumps are fine, though that does not dismiss other fuel related faults that could be creating your rough running condition. Is the mixture noticeably rich or lean? Have you done a vacuum check?

I've been battling a vacuum leak for a while now with my '84. Car began to run very poorly more and more often. It's also worth noting that it behaved in a cycle much the way you describe and the vehicle was going between extreme lean and rich fuel mixtures. Turned out that two of the intake gaskets were creating the leak. Never overlook your intake seal as a potential fault with a rough running 928. So often, the cause is a simple, easy-to-fix vacuum leak.
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1978 928 5 speed fuel pump delivery test. 25 Apr 2010 10:12 #234


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Thanks. I've been looking for obvious, visual intake leaks and don't see any. My recently rebuilt warm up regulator had below accepted range control pressure readings = rich running condition. Have sent faulty WUR back to IMC for new rebuild. I imagine I have to have car running to do vacuum test?
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1978 928 5 speed fuel pump delivery test. 25 Apr 2010 12:02 #236


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Yes. Have you tried spraying starting fluid around the manifold and vacuum lines to spot leaks? I don't know if you're familiar with the Nichols 928 tips page, but someone once posted a tip for creating manifold pressure without having to run the vehicle by forcing air through the AFM and then listening for air leaks. Though, I have not done that, it sounds like it would be very effective as the noise from the vehicle running would not be a distraction.

Here's the link from Greg's site... http://www.nichols.nu/tip367.htm
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1978 928 5 speed fuel pump delivery test. 25 Apr 2010 13:39 #238


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I have sprayed with no results. I have a '78 928 with K basic CIS--no afm. but I may be able to put vacuum pump on intake plenum and look for reduction in vacuum. but I am quite sure WUR is the problem.
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1978 928 5 speed fuel pump delivery test. 25 Apr 2010 17:14 #239


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My limited knowledge of K-jet systems tells me that your suspicion is probably right. The WUR doesn't seem to respond well to sitting for very long as I presume your car did. Eliminating the easiest and/or cheapest possibilities first is always the way I work, though.

Just out of curiosity, what were your findings when you checked the system's control pressure?
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1978 928 5 speed fuel pump delivery test. 25 Apr 2010 17:28 #240


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Control pressure warm without vacuum was 2.3 bar should be 2.8-3.2. Control pressure warm with vacuum was 3 bar should be 3.4 - 3.8.
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