Rest Pressure

[Bitsyncmaster]

My rest pressure is dropping to 10 PSI in 50 minutes from shutdown. In the past it would hold 2 to 3 hours. Doing some testing I noticed a few things.

My accumulator does not clank anymore. It did have a clink sound as the spring inside it released the slowly dropping rest pressure.

My system always would instantly drop to about 40 PSI when I shut off the fuel pump. With the pump running I get 74 PSI. I'm guessing this is normal and must be something in the fuel distributor to cause that. If not then I would expect the rest pressure to slowly drop from the 74 PSI when it ran.

Another thing I see is pressure drops slowly from the 40 PSI start down to 33 PSI and then it starts dropping much faster.

I'm going to track this down. I built an electronic timer to count elapsed time until rest pressure drops below 10 PSI.

I replaced the PPR O-ring with a new one and no change. I ordered a new check valve and will test again when I get that installed. My gut feeling is the accumulator is failing (about 20 years old now).

[Rich]

My system always would instantly drop to about 40 PSI when I shut off the fuel pump. With the pump running I get 74 PSI. I'm guessing this is normal and must be something in the fuel distributor to cause that. If not then I would expect the rest pressure to slowly drop from the 74 PSI when it ran.


Side note not directly tied to your car's rest pressure issue:

Yes, the K-Jet system is designed so the pressure drops to about 3 bar (~40psi) and then holds that pressure for some time after engine shutoff. As the injectors all have an opening pressure of 3.5-4.1 bar this feature keeps them from bleeding fuel after shutdown.

[Bitsyncmaster]

Side note not directly tied to your car's rest pressure issue:

Yes, the K-Jet system is designed so the pressure drops to about 3 bar (~40psi) and then holds that pressure for some time after engine shutoff. As the injectors all have an opening pressure of 3.5-4.1 bar this feature keeps them from bleeding fuel after shutdown.

I did run a test some time ago. Was holding rest pressure and I removed one injector line. It did nothing as far as holding rest pressure. So if your mixture is adjusted correctly, leaking injectors do not cause loss of rest pressure. Now if you see drops during rest pressure, you FD or mixture setting is at fault.

[Bitsyncmaster]

I just thought of a way to measure the volume of the fuel accumulator. With the standard pressure tester, leave the hose going to the WUR in a jar. Jumper the pump to get primary pressure, pull the jumper and then open the valve on the pressure tester. Measure the fuel in the jar and you have the volume of the accumulator. My primary pressure instantly drops to 33 PSI and then slowly climbs to 37 PSI. That is where my rest pressure starts to slowly drop.

Right now I have my electronic pressure transducer in that WUR hose location. So I will do this test after I finish my rest pressure testing. I do have the stock size accumulator in my car.

[Bitsyncmaster]

Well it's not the check valve. Replaced it with a Viton one. Don't think they had Viton when I first bought the original check valve.

Anyway still have the same problem. Slowly drops from the 38-39 PSI start of rest pressure but when it hits 33 PSI it drops rapidly. Two things I can think maybe the cause.

1). The spring in the accumulator is broken. It does not make the clunking sound anymore.
2). An O-ring in the FD is faulty. I did rebuild my FD a few years back but never put new O-rings into it.

[Bitsyncmaster]

I pulled the fuel accumulator. The vent side was completely full of fuel. Not sure if this is normal. Vent is attached to a shared tank return line so it may have filled from returned fuel from that line.

[mhanch]

That's not normal according to everyone i asked before. any fuel in that vent side usually means the internals are shot. but a new one isn't too expensive.

[Bitsyncmaster]

That's not normal according to everyone i asked before. any fuel in that vent side usually means the internals are shot. but a new one isn't too expensive.

Have you seen the prices lately?

https://store.classicdmc.com/100519-fuel-accumulator-w-pad.html

[Timeless]

Have you seen the prices lately?

https://store.classicdmc.com/100519-fuel-accumulator-w-pad.html
Whoa - quite a jump. Glad I'm EFI now.

[mhanch]

https://www.kmotorshop.com/en/article-detail/view/290576/pressure-accumulator-fuel-pressure-0438170029-bosch-156013-7700688585-91503996


Not sure about shipping. prices are up.

[eseethal]

I had this with a new accumulator too.


I pulled the fuel accumulator. The vent side was completely full of fuel. Not sure if this is normal. Vent is attached to a shared tank return line so it may have filled from returned fuel from that line.

[Bitsyncmaster]

While waiting on some pluming parts to arrive to install my new (not stock) fuel accumulator, I was cleaning up my software for my test gauge. This is using an existing unit I built for my parts curing oven. No hardware changes just software since the temperature sender and my pressure sender both provide a voltage with a 5 volt power connection. I need to see if I can store both routines into my micro so I don't need to take the back cover off to load each software program.

Here is the unit working with my pressure sender attached to my compressor air supply.

[Bitsyncmaster]

I got both the rest pressure software and oven software to fit in my 4K word CPU. I think I will remove the pressure gauge on my fuel pressure test setup and just use the pressure transducer. Only drawback is your need AC power. Nice thing is you don't have to tap on the gauge if your looking for accurate pressures. Don't need to remove the back cover anymore.

[Bitsyncmaster]

Here is what I'm trying. Since the OEM accumulators seem to be in short supply, I'm trying another one. This one is the same physical size but has the same size mating connection for both. Will still connect to the OEM elbow going to the hard line. The connection from the hose now needs to be a 14 mm X 1.5 "ball" connection. If you have a damaged hard line you can also use a hose connection to replace the stock elbow.

You could use most any accumulator. Even one with one port you would need to install a "T" in the lines. I guess using the accumulator as the "T" removes one pipe joint. Fuel really does not need to flow through the accumulator, it just needs to supply rest pressure.

[Bitsyncmaster]

Just installed the new accumulator. What I do is tighten both the hose connection and the elbow onto the accumulator with it just hanging. Then the return hose also with it hanging (I put a longer hose on that also). Then it is a fight to get that elbow started onto the hard line. Then the fight to get the 4 bolts into the hold down clamp. Then there is enough room to tighten that elbow to hard line nut with a standard length wrench.

Anyway, my clunking is back (ya-ho). Jumping the fuel pump it really took longer then I expected for the accumulator clunking to stop. Anyway, still 73 PSI primary pressure. Still instantly drops to 35 PSI when I shut off the fuel pump and then slowly climbs to 39 or 40 PSI. Now I'm running my electronic timer for rest pressure time.

Still holding 37 PSI after 30 minutes.

[dn010]

Under the center console is an access panel cut into the underbody that you can open to get access to the lines from above. I found it easier to drop the T and elbow down off the main line, through the slots in the frame and deal with the hoses with the accumulator out of the car. That was a very long time ago when it probably seemed much easier anyway.

Do you plan to open the old accumulator to see what you were dealing with?

[Bitsyncmaster]

Under the center console is an access panel cut into the underbody that you can open to get access to the lines from above. I found it easier to drop the T and elbow down off the main line, through the slots in the frame and deal with the hoses with the accumulator out of the car. That was a very long time ago when it probably seemed much easier anyway.

Do you plan to open the old accumulator to see what you were dealing with?

I may open the old one. But I'm guessing is just got a leak in the bellows since the return side was full of fuel.

[Bitsyncmaster]

Still have 12 PSI after 5 hours of rest pressure. My setup is still running. What it does is zero the elapse time counter above 60 PSI and count time above 10 PSI. That way it auto starts the timing right when you shut the fuel pump off. This has far exceeded any timing I have done since I owned the car. It never got above 3 hours to 5 PSI. Now I can even disable my RPM relay hot fix if I want, but really no reason to disable it.

I just randomly selected 10 PSI as a stop for the rest pressure timing. Not sure if 10 PSI would stop fuel vapor.

I want to do some testing with the fuel pressure also feeding the WUR. I would guess on a hot engine that would not affect rest pressure time because the control pressure should stay above the rest pressure. Doing that test would be more meaningful because that is how the engine runs.

[Bitsyncmaster]

First test was 5 hours and 31 minuets. Still holding 3 PSI after 13 hours.

[Bitsyncmaster]

I just started the test with the valve open (pressure to WUR) cold engine (74 F). Note it took about 40 seconds running the pump to have the accumulator stop clunking. With the pump running I was getting 28 PSI. When the pump stopped running it climbed back up to 39 PSI just like it did when the valve was closed. So maybe it does not matter if your running rest pressure time with the valve open or closed.

Don't know why it takes so long for the accumulator to stop clunking.

I do have to close the valve with the pump running to have my gauge go above my 60 PSI reset pressure. I then open the valve after the pump is stopped. But I think the primary pressure would still be 73 PSI if I was measuring at the fuel input line at the FD.

[Bitsyncmaster]

Rest pressure time with valve open (feeding WUR) on a cold engine is 3 hours and 15 minuets. The workshop manual shows an O-ring buried in the PPR which I have never changed. I will look into getting that changed.

[Bitsyncmaster]

I'm thinking there is an orifice in the accumulator so it slows the fill time. That would make sense so you get full pressure to the FD very quickly. I will have to cut the old accumulator open to check that.

In any case, if your doing rest pressure checks, make sure your fuel pump runs for one minuet so your sure the accumulator is full.

[Bitsyncmaster]

I could not get that 5 hr test to repeat. I now get around 3.5 hr. Don't know why. Anyway, I'm playing with a softer O-ring for the PPR. Softer should seal better at lower pressures. I should have made my test set plot the PSI on a curve with time. I would need to use a graphics display or add a connection to my lap top.

The accumulator I now use is not the OEM spec number. My current one has an orifice to slow the filling of the accumulator. The OEM one does not have that orifice. It does not matter with or without as far as operation in our car but it does make my RPM relay hot start fix not work. I would need to bump the prime charges up to 40 seconds long to get that fix back. The orifice unit would also make the key on prime less needed since you would get full pressure almost instantly.

[poidog]

The DMC spec accumulator is available from vendors on eBay at the regular price and not the grossly inflated price the vendors indiscriminately jacked the price up to. Just search Bosch 0438170029. Buy a spare to keep on the shelf for next time.

[Bitsyncmaster]

I found the problem with the rest pressure when the valve is open (WUR has flow). You really should be checking the rest pressure time this way since that is how your engine holds rest pressure when turned off. There is a second valve in the PPR that shuts off the WUR return line when the fuel pump is not running. What I found was that O-ring on the outside of the PPR was a little to small and not sealing the PPR to the FD case. I did not have one a little bigger so I forced grease into that O-ring slot and retested rest pressure time. I think I bought the O-ring kit from Hervey about 20 years ago. I guess he used the wrong size O-ring. I'm not sure if that grease will hold up over time, so I placed an order for some O-rings sized a little larger.

Test one valve open, 3:29:35 (times are hours: minuets: seconds) Time is from pump shut off until pressure drops below 10 PSI
Test two valve open, 3:28:55
Test three valve closed, 3:38:28
Test four valve closed, 3:40:38

Doing a rest pressure time with the valve closed does not test that WUR return valve. So you may want to do that if troubleshooting a short rest pressure time.

I still don't know why my first test showed 5 hours. The system was empty since I just replaced the accumulator.

[Bitsyncmaster]

While waiting for my O-rings, I ran a control pressure check. Another owner was asking about control pressure.

My engine was at 73 deg. I jumpered the fuel pump and control pressure is 28 PSI. I put the RPM relay back in and start the engine. After 30 seconds of running, control pressure is 37 PSI. After 60 seconds of running control pressure is at 47 PSI which is where it stays bouncing from 47 or 48 PSI idling for 7 minuets.

Hitting the gas does not seem to affect control pressure. I think the thermal valve in the valley has not yet opened.

Is this normal? I don't notice any problems driving the car right after a cold start up.

Note my primary pressure is 75 PSI. That softer Viton O-ring bumped the primary pressure up 2 more PSI.

[Bitsyncmaster]

Just as I thought. After running that control pressure test, the rest pressure time was 2 hrs. 3 minutes. Waiting on the larger O-rings.

[Bitsyncmaster]

I've been doing a lot of rest pressure timing. What I have found if I run the engine for 5 minuets to start the timing, I get a lot less rest pressure time. It's like 40 % shorter. I'm thinking something in my FD is causing that but not sure if that is normal. I guess I'm the first to do extensive rest pressure work.

Anyway, just got my new O-ring for the outside of the PPR. What I'm using now is a 2 mm x 7 mm V75 FKM O-ring. I do feel it compress when installing the PPR. The old O-ring had no compression, the PPR just slid in an out with no force. Running a rest pressure time now.

Anyway, if you need O-rings, use the O-RING STORE LLC. Not only real inexpensive but shipping was only 3 days. I paid 8 cents each. Had to but 100 of two sizes to get my order past $10 which is their minimum order.

Also to note. I installed a new crush washer for the PPR. The new one was 0.010 thicker but my primary press when from 73 PSI to 77-78 PSI. I'm guessing it crushed more when I tightened it.

Also to note. I now use the CSV port for measuring my rest pressure. It lets you read primary pressure with the engine running.

[Bitsyncmaster]

The new O-ring and crush washer did not change anything.

But I went back to think about all the different testing I did and I thought of one more thing to test. I'm 90% sure this will not only solve my rest pressure differences but will also help reduce the sensitivity of my centering the lambda system. My car has always been, just a touch of the mixture screw would send the dwell readings swinging a lot.

Will get back to this after this last rest pressure time test. Hopefully it adds many hours.

[Bitsyncmaster]

I made a couple banjo fittings to work as a plug. I brazed the hole.

With the WUR, CSV and FV plugged, I ran multiple rest pressure times and all ran 4.0 hours give or take 5 minuets.

WUR open and got the same 4.0 hours. I do think my outside O-ring and new crush washer fixed this test. But my previous testing was not so controlled.

WUR and CSV open still got the same 4.0 hours.

WUR, CSV and FV open ran 3 hrs. and 55 minutes. Ran that test twice. I did run the FV for a minuet with the fuel pump running to simulate and engine run.

Now started the engine and ran for 5 minuets. Rest pressure time resulted in 2 hrs and 4 minutes. I would not think that drop in time would be normal. So it must be something in the FD not sealing immediately. I do have an FV rebuild kit but not sure if I will rebuild it. Rest pressure time is not that important but I would like my dwell setting to be less sensitive to the adjustment. My FD is not the stock unit. It is the one with the flow adjustments for each cylinder. I do have some used FVs coming in soon. Maybe that was also changed to a non-stock unit.

Does anybody know the BOSCH part number of the stock FV?

Side note: I had my car up on ramps in the front and 6x6 blocks under the rear wheels to change my accumulator. I removed the blocks and the engine would not stay running. I had a little over 1/8 tank indicated. Pushed it off the ramps and went to fill it up and it took 11.8 gal.

[Bitsyncmaster]

I couldn't resist. I saved the rest pressure at one minute intervals in EEprom. This is after a 5 minute engine run. PSI on left and minutes on bottom.

[Rich]

Nice to see the data. It's good news depending on your criteria.

According to the factory rest pressure test criteria your system passed the test by a wide margin.

[Bitsyncmaster]

I did a lot more testing to see what affects rest pressure time. I did find increasing my hot control pressure from 46 to 52 PSI did bump the running engine time from 2 hours to 3 hours. Also found that run engine time is affected by your mixture setting adjustment. So that does lead me to believe something in the fuel distributor is mostly what determines the rest pressure time. My mixture adjustment is very critical. Less then 1/16 turn can make it go from full dwell to min dwell. I do have a non stock fuel distributor. It is one with adjustable volume on each cylinder. I wonder if it is just more or less fuel flow than the stock fuel distributor. Not sure how much those adjustments could make for the volume of fuel. Maybe I could adjust them all for more or less fuel.

Anyway, attached my latest rest pressure graph.

[Bitsyncmaster]

I've decided to install the stock fuel accumulator. I ran a test dropping my primary pressure 5 PSI and that showed me the accumulator I have now has a spring rate to high. The rest start pressure dropped and the time got very short.

[mark w]

Dave, what Fuel pump setup are you running?

[Bitsyncmaster]

Dave, what Fuel pump setup are you running?

The Tahoe pump setup. I did have to rotate the pump for the float to clear the tank wall. It is interesting I found my primary pressure changes a little with voltage the pump gets. With just a fully charged battery I get 71 PSI. With engine running I get 77-78 PSI. My car has 14.5 volts with engine running. I guess the flow rate is affecting the PPR.

You can read primary pressure with engine running using either the CSV or FV ports.

[Bitsyncmaster]

Got my new accumulator. Happy it is a BOSCH unit made in Germany same part number. Only difference from the old one is it's not that drab green color.

[Bitsyncmaster]

Installed the new (stock) fuel accumulator. Note, the working removed one was full of fuel on the vent side. It pulls in fuel from the return line when it looses rest pressure.

So far, I think it performs just like the other one. Took a long time to stop the clunking just like the other one. I think the small bypass line on the return is what acts like an orifice to slow how fast the accumulator fills.

[Bitsyncmaster]

The new accumulator graph. Engine run up to temp then started rest pressure recording. Old graph also. So pretty much the same thing for rest pressure but new one held longer.

[Bitsyncmaster]

Getting ready to wrap up this testing. Summery of things I've found:

1) The fuel accumulators do take about 40 to 45 seconds of pump running to fully charge them. The RPM engine prime does bring up rest pressure to 32 PSI and that will hold for 15 to 30 minuets without the help of the accumulator. Takes 2 more primes to keep moving that rest pressure up one more PSI. You will eventually hit the 41-42 PSI and it will not go above that.

2) If you have your mixture set a little rich (higher dwell numbers, mix setting moved CCW) that will reduce rest pressure time.

3) I think BOSCH is making new fuel accumulators now. My old one has a 050 stamped on it and I bought that one in 2005-2006. My two new ones have a 250 stamped on them. So I'm guessing that number is a date code.

4) I think the accumulator is what sets the start of rest pressure PSI. So my guess is the spring inside it starts to compress around 32 PSI and is fully compressed around 42 PSI. There is something acting as an orifice which slows the time it takes to compress. The vent line has a very small diameter so it may be acting like an orifice. I should cut my old one open to see if the orifice is inside.

[Bitsyncmaster]

I'm thinking my original fuel accumulator was not bad. I never had a starting problem but I noticed the rest pressure seemed to short. Never knew it took 40 to 45 seconds of pump running to fully charge the accumulator.

I wonder how many other owners have replace theirs for the same reason. I guess if you have a hot start problem then it was bad.

[Bitsyncmaster]

Some interesting data I got when doing my electronic PPR testing. My rest pressure time has really gone up quite a bit. I noted that before with the short times my pressure would fall off quickly below 30 PSI. Now it falls off slowly. My last engine run rest pressure took 3 hours and 49 minuets to drop to 10 PSI. But then it will hold another few hours to drop to 3 PSI.

All I can think of is the accumulator spring was hanging up so it stopped helping below 30 PSI. This new accumulator clunks 14 times in the 40 second prime time at 75 PSI. I never counted the clunks when it released the rest pressure but I guess it was less. Don't know why the accumulator spring has to "clunk" when it moves.