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Strange fuel pressure in 1982 928s Euro

 
Old 05-29-2019, 08:39 AM
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camelman
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Default Strange fuel pressure in 1982 928s Euro

Iíve been working at running issues with my car for a year. It seems that every time I fix something, another aspect of operations gets worse. I recently sent my FD to Salvox for a rebuild, but got oddly low fuel pressure readings upon reassembly. Then, my 944S2 blew a head gasket due to a badly corroded cylinder head and I stopped working on the 928s. The 944S2 is now repaired, so back to the 928s.

I found very low fuel pressure and very poor running with some black smoke after rebuilding the FD at Salvox. I thought my fuel pressure tester gauge was bad so I tried another. With the replacement gauge, I still registered low fuel pressure. I noticed the fuel pressure gauge needle was “wiggling” around 5-10psi with the pump jumpered on, so I kept the pump on. Then, after about 15 seconds I heard a “pop or ting” somewhere in the engine bay and the fuel pressure jumped up to 68psi. The pressure drops very quickly when the fuel pump is turned off with a ticking sound occurring from the back of the engine that corresponds to the gauge needle jumping, and then falling again.

The fuel pressure was measured between the WUR and FD. Any idea what could cause that response? It definitely seems like there was a blockage that freed up.

Iím waiting on my Bosch fuel injection book to arrive, but looking for advice regardless. What could cause a fuel pressure affect like this? Bad WUR, blocked supply or return lines, bad accumulator? Is there a separate fuel pressure regulator from the WUR? I feel like Iíve thrown a lot of time and money at this car with poor results, and I want to make sure Iím chasing the correct thing.
  • new fuel pump and filter
  • new in-tank sock filter
  • new fuel lines and hoses around the fuel tank
  • fuel tank removed and rinsed
  • rebuilt FD by Salvox with all injectors firing simultaneously
  • WUR cleaned up with no clog in the filter screen and rebuilt with new gaskets
  • new MB injectors
  • cleaned screen in the fuel supply line connector to the FD
  • brake booster vac line pulled and plugged
  • new vac lines throughout engine
  • new intake gaskets
  • Air plenum cleaned and separated with new gasket

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Old 05-29-2019, 09:13 PM
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Anybody? Did I say something stupid?
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Old 05-29-2019, 09:51 PM
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Hi. I too have a 82 Euro S that I rebuilt from trash. Thread here http://forums.pelicanparts.com/porsc...re-begins.html and all my posts on it on this forum. Research it all. I chased my tail on bad running problems for a long time and my smoking gun was the Mercedes Benz fuel distributor I was trying to use. After I found the right one (correct part #) that came original on the car I was basically all fixed. I sent my WUR to Australia in exchange for the correct one for the car and itís all smooth now. My advice is research the ones that have been there, make sure you have the correct part #s (Mercedes injectors are ok). Also get a permanent air fuel ratio gauge in the cockpit). Gunar
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Old 05-29-2019, 11:30 PM
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http://k-jet.biz/

these guys know what they are doing. Highly recommended
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Old 05-29-2019, 11:51 PM
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Lots of info for you about testing under ďtestsĒ on their websi

te
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Old 05-30-2019, 10:18 AM
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I have had issues with other rebuilders, including my own doing, for distributors and found this guy to have the best ones. Never had an issue with one of his and he usually has ours in stock.
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Old 05-30-2019, 06:43 PM
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Thanks all! Youíve given me plenty more to work with.

i think my FD is operating correctly now, but my WUR might be an issue. Will chase it down soon.
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Old 07-10-2019, 03:35 AM
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I finally got back into the S tonight. It starts and runs, but responds poorly to throttle. Fuel pressure with the engine and WUR cold is 68psi system, and roughly the same for control. Pressure doesnít drop quickly now, but control pressure is just too high.

I noticed that fuel pressure jumped to 90psi after I futzed with the gas pedal for a bit to get the engine revving. I had to lightly push, then let up, and repeat, to get the engine to finally rev. It revved to 3k, but then died. It was after that experiment that I noticed the 90psi.

Iíd like to check the fuel return line, but am not sure which one it is. I also think my WUR is causing problems. thoughts?

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Old 07-10-2019, 12:07 PM
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A few more notes for clarity:

~ I only saw 90psi once
~ when I first started testing, I saw 68psi system and roughly 40psi control with all systems off other than a switch to the fuel pump
~ a regular clicking is audible from, I believe, the FD that regulated the pressure to 40psi
~ after running the engine a few times the clicking mostly stopped and the control pressure remained at 68psi

It seems to me that something is wrong with the pressure regulator in the FD. Recall that the fuel pressure was below 10psi after the FD rebuild until a loud click from the FD happened and fuel pressure shot up. I had the FD rebuilt by CIS flowtech and they have a 1 year warranty.

I also want to check the fuel return line, but am not sure which one it is.

Thanks,
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Old 07-10-2019, 07:39 PM
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The return line(outlet)from WUR is the one going sideways to the car's left side, around to the return connection on the FD. Get a copy of Ben Watson on Bosch FI - has good troubleshooting . With pump jumpered and pressure gauge fitted, carefully crack open the WUR outlet a little (with rags and extinguisher handy!) - if pressure drops and much fuel comes out, the WUR is NOT causing high control; if little fuel comes out, pressure stays high, WUR is blocked. I have had good results forcing carb cleaner backwards through the WUR with a syringe or similar. Also look for lint blocking the gauze filter in the WUR inlet . I also jumpered the pump for an hour or more with tank loaded with cleaning oil to try to flush crud out into the filter.
jp 83 Euro S AT 57k
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Old 07-11-2019, 10:28 AM
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With a rebuilt FD, you are going to have to re-adjust the A/F mixture as well and possibly need to reshim the side FD screw. How are you measuring for that that? With out some sort of CO analyzer or gauge set, you are only guessing...
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Old 07-11-2019, 01:08 PM
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I heard back from CIS flow tech and Larry thinks it might be a clogged return line too. I'll check that this weekend starting at the WUR. What should I expect in the return line? Should I just be able to blow air through it?

I've been adjusting the F/A ratio by observing the exhaust and listening to the engine. The initial setting was found by pressurizing the fuel system and turning the AF screw until the injectors started to sing, and then backing off 1/8 of a turn. A friend has a wide band O2 sensor that I'll use for final testing, but only once I get the fuel pressure situation remedied.
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Old 07-11-2019, 03:40 PM
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Originally Posted by camelman View Post
I heard back from CIS flow tech and Larry thinks it might be a clogged return line too. I'll check that this weekend starting at the WUR. What should I expect in the return line? Should I just be able to blow air through it?

I've been adjusting the F/A ratio by observing the exhaust and listening to the engine. The initial setting was found by pressurizing the fuel system and turning the AF screw until the injectors started to sing, and then backing off 1/8 of a turn. A friend has a wide band O2 sensor that I'll use for final testing, but only once I get the fuel pressure situation remedied.
That's a good start but the A/F is hyper sensitive. 1/8 of the a turn could adjust the stoichiometric reading one point. Until you know exactly where you are at, you could running 11s or 12s...
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Old 07-12-2019, 05:59 PM
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Larry from CIS Flowtech called me today and talked me through his thoughts on the problem. I'm going to check the entire return line system this weekend, and hopefully get some measurements on volumetric fuel flow. It sure takes a while to learn this system and to get comfortable working on it!
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