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Thread: Finding cause of loss of rest pressure

  1. #1
    Senior Member Bitsyncmaster's Avatar
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    Question Finding cause of loss of rest pressure

    I had a local owner come over to my place so I could help him with his car. He has a hot start problem so I put the pressure gauge on his engine and sure enough it looses rest pressure in about 10 minuets. He said his dad replaced the fuel accumulator so I pulled the PPR to replace that small O-ring. The old one looked good and replacing the o-ring did not change rest pressure time. Pulled the cover over the fuel pump thinking the check valve was bad. He has the new DMCH pump and it has the check valve in the correct line. So I'm thinking the accumulator is bad or the injectors leak. I did do a minor mixture adjustment so that setting was not far off and it did not change rest pressure. I sold him my hot start relay but expect him to return someday so I can test the injectors (It's to cold today and getting late).

    Anyway. This engine is very strange. The idle motor is some horizontal unit from some other car. The throttle cable reel is from some other car also. There is no ignition advance solenoid. I'm guessing these changes are all some previous owners modifications or the engine was pulled from a Volvo. Owner said car was bought from Albuquerque New Mexico.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  2. #2
    My frame has two coats of wax Michael's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    I had a local owner come over to my place so I could help him with his car. He has a hot start problem so I put the pressure gauge on his engine and sure enough it looses rest pressure in about 10 minuets. He said his dad replaced the fuel accumulator so I pulled the PPR to replace that small O-ring. The old one looked good and replacing the o-ring did not change rest pressure time. Pulled the cover over the fuel pump thinking the check valve was bad. He has the new DMCH pump and it has the check valve in the correct line. So I'm thinking the accumulator is bad or the injectors leak. I did do a minor mixture adjustment so that setting was not far off and it did not change rest pressure. I sold him my hot start relay but expect him to return someday so I can test the injectors (It's to cold today and getting late).

    Anyway. This engine is very strange. The idle motor is some horizontal unit from some other car. The throttle cable reel is from some other car also. There is no ignition advance solenoid. I'm guessing these changes are all some previous owners modifications or the engine was pulled from a Volvo. Owner said car was bought from Albuquerque New Mexico.
    I think the next step would be to pull the FD and check the plunger play. Also, are there not 3 o rings on the PPR to replace? I know I am doing mine about every 2-3 years from now on because they are cheap and easy to replace and a bad one can look good, especially when it's in place on the plunger.

  3. #3
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Michael View Post
    I think the next step would be to pull the FD and check the plunger play. Also, are there not 3 o rings on the PPR to replace? I know I am doing mine about every 2-3 years from now on because they are cheap and easy to replace and a bad one can look good, especially when it's in place on the plunger.
    There is a larger O-ring and a copper sealing washer. When I was adjusting my primary pressure I found you did not even have to tighten the PPR on and it held rest pressure fine so I don't think the other O-ring is needed to hold rest pressure. His air plate did seem a little tighter than mine. Mine will almost bounce with no rest pressure. But the rest pressure loss rate is very consistent.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #4
    Administrator Ron's Avatar
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    I think he meant the 3 o-rings shown here:
    PPR.jpg

  5. #5
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Ron View Post
    I think he meant the 3 o-rings shown here:
    PPR.jpg
    I think that drawing shows a different type of PPR. The one in my car and the car I just tested, the PPR has only one spring and the "push valve" is on the left side.

    I was thinking, would it show me if the leak was before the injectors if I leave my pressure gauge set to primary pressure (valve closed)? I don't think it does since when the pump is shut off the pressure immediately drops to around 40 PSI.

    What I think I need to do is make a way to attach a gauge onto the disconnected line from the accumulator. I would need to only run the pump until I get 75 PSI or less since there is no limit other than the pump for pressure.
    Last edited by Bitsyncmaster; 01-31-2021 at 06:06 AM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #6
    My frame has two coats of wax Michael's Avatar
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    Quote Originally Posted by Bitsyncmaster View Post
    There is a larger O-ring and a copper sealing washer. When I was adjusting my primary pressure I found you did not even have to tighten the PPR on and it held rest pressure fine so I don't think the other O-ring is needed to hold rest pressure. His air plate did seem a little tighter than mine. Mine will almost bounce with no rest pressure. But the rest pressure loss rate is very consistent.
    I know there are at least 2 O-rings and one washer on my PPR, but I think I read that there is a third O-ring inside one of the carriers but I have never replaced it, just 2 rubber rings and single copper washer.

    I wonder if the PO got a Volvo distributor on his car or some other cross referenced distributor? Maybe the PPR is slightly different on it? Did it have the frequency valve in place? Maybe his has the wrong thickness shim on it?

    I would still look at the plunger on the FD first and make sure it's not sticking.

  7. #7
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Michael View Post
    I know there are at least 2 O-rings and one washer on my PPR, but I think I read that there is a third O-ring inside one of the carriers but I have never replaced it, just 2 rubber rings and single copper washer.

    I wonder if the PO got a Volvo distributor on his car or some other cross referenced distributor? Maybe the PPR is slightly different on it? Did it have the frequency valve in place? Maybe his has the wrong thickness shim on it?

    I would still look at the plunger on the FD first and make sure it's not sticking.
    Yes it had the lambda system that works because I was able to set the dwell. It actually was a little more stable with dwell then I've seen.

    Primary pressure was 69 PSI which I've seen as common on our D if you have not shimmed it. Control pressure on a warmed up engine was 48 PSI which is close to the 50 PSI I normally see but it was pretty cold yesterday.

    I'm machining a fixture that will let me attach the line that comes from the fuel accumulator. Just a simple block of aluminum with the 12 mm thread on one end. I use the hollow bolt from the removed line and a nipple on the other end I can install the hose to my pressure gauge. So I will quickly jumper the pump on then off and watch how lone it can hold pressure (around 75 PSI). That will tell me quickly if the loss is with the FD, injectors, accumulator or fuel pump.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #8
    Administrator Ron's Avatar
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    Are you sure?? Pre 1978, there were no push valves. Post 1978 used a push valve (as in the pic above) located on the right side of the FD when installed on a DeLorean. (You can look down the bore when standing at the rear.They were added to control the return of fuel from the control pressure regulator. With the engine off, the larger spring closes the relief plunger and the smaller spring closes the push valve, sealing control pressure in the system.

    Anyway, if I understand the question, I don't think you could tell that way because the primary and control circuits are connected via the restrictor passage in the FD.

    Other thoughts:
    "If residual pressure falls below the minimums with the gauge valve open, the leak is probably in the control-pressure circuit, at the push-valve 0-rings. See Fig. 5-16. If it is an older system without a push valve, the leak is probably at the control pressure regulator." (BOSCH K-Jet)

    The cold start valve could be leaking (rare).

  9. #9
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by Ron View Post
    Are you sure?? Pre 1978, there were no push valves. Post 1978 used a push valve (as in the pic above) located on the right side of the FD when installed on a DeLorean. (You can look down the bore when standing at the rear.They were added to control the return of fuel from the control pressure regulator. With the engine off, the larger spring closes the relief plunger and the smaller spring closes the push valve, sealing control pressure in the system.

    Anyway, if I understand the question, I don't think you could tell that way because the primary and control circuits are connected via the restrictor passage in the FD.

    Other thoughts:
    "If residual pressure falls below the minimums with the gauge valve open, the leak is probably in the control-pressure circuit, at the push-valve 0-rings. See Fig. 5-16. If it is an older system without a push valve, the leak is probably at the control pressure regulator." (BOSCH K-Jet)

    The cold start valve could be leaking (rare).
    It was the same PPR that I have but mine is an adjustable (non stock) FD. All I see when I pull it out is the spring, shim on a metal rod. Then if your lucky the piston comes out which has the small O-ring on it's end. It may come apart more since that end cap looks like something it threaded into it but I've never taken that apart. When I do the pressure time test on my car I will pull the PPR and post a photo.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  10. #10
    Administrator Ron's Avatar
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    The post 1978 and adjustable have one that looks like this (but the small o-ring you mentioned is missing):
    PPR o-rings.jpg
    To get to the push up o-ring, push the keeper ring back (see arrow) to expose a tiny snap ring. Remove it and the rest should be obvious...

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