Fuel pressure

[deloryan]

I’m trying to diagnose an issue where engine really stutters on acceleration. It is idling okay but doesn’t want to rev. My fuel pressure on both control and primary is low but adding shims is not helping. I replaced my fuel pump with the new style thinking maybe the old pump just wasn’t putting out enough pressure.

I’m checking a few other things and am wondering if my WUR has a dirty screen or is this the screen itself. It doesn’t look like other pictures I’ve seen.

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[Helirich]

Have you checked/replaced the fuel filter. I haven't had much luck with the shims either.

Is this a problem that just started? Or are you working on a car that's been sitting?

There are several screens stacked up in there. You might want to remove them with a pick. If the screens are clogged, you will have high control pressure.

[deloryan]

I replaced the fuel filter today. I’m only getting 5 bar directly at the fuel pump. And about 4.5 bar where the line enters the fuel distributor. So I think that tells me the fuel pump is not strong enough, even though it is new. It’s the AC Delco MU1777. And the accumulator, filter, and lines are restricting a bit of pressure. So it isn’t going to matter how many shims go in if the base pressure isn’t even high enough.

Car sat the last two summers. I drained and cleaned the fuel tank, too.

[Ron]

Something seems wrong here -- The pressure at the FD should match the pump pressure. (The accumulator, filter, and lines might restrict flow, but not pressure to the FD).

Also, with 4.5 Bar at the FD, the control pressure being low wouldn't have anything to do with the pump...
So I would suggest checking for leaks, the fuel line routing, and double checking the pressure at the pump (and replace it if not >6 Bar before going further).

67137

[FABombjoy]

You can also leak past fuel the accumulator seal to the return line which would present similarly. That is, higher pressure at pump than at fuel distributor. This is a failure mode not discussed much but happens with time.

[deloryan]

You can also leak past fuel the accumulator seal to the return line which would present similarly. That is, higher pressure at pump than at fuel distributor. This is a failure mode not discussed much but happens with time.

I’ve ordered an accumulator as this one is about 15 years old, so could be an issue. I also think something must be wrong with the new fuel pump itself since I’m not seeing great pressure there. Battery is new and voltage is good at the fuel pump.

[deloryan]

Something seems wrong here -- The pressure at the FD should match the pump pressure. (The accumulator, filter, and lines might restrict flow, but not pressure to the FD).

Also, with 4.5 Bar at the FD, the control pressure being low wouldn't have anything to do with the pump...
So I would suggest checking for leaks, the fuel line routing, and double checking the pressure at the pump (and replace it if not >6 Bar before going further).

67137

Not seeing any leaks and nothing on the ground. I haven’t messed with fuel lines(except the one for pressure testing) so they are all routed correctly. I agree, it seems the fuel pump might be the issue even though it’s new. Any recommendations on a better replacement? I don’t want to go back to the old style.

[Jonathan]

Double check the two hose connections on the fuel pump, maybe they got switched around?

[82DMC12]

No reason to change the accumulator unless you fail the "rest pressure test". Also you can remove the small line on the rear of the accumulator, plug the rubber hose, jump the RPM relay, and see if fuel is dribbling out of the accumulator where the hose used to go. If you are dripping, change the accumulator. If it's not dripping, it's fine, and your problem is somewhere else.

Where did you get the part # for the fuel pump? I have a new pump from a vendor but it's a Delphi.

Change the fuel filter, check the accumulator as I explained, and then re-test the primary pressure. Check the shop manual to see how to put the pressure tester in line with the CPR. It's easy to get it backwards in regards to the bypass that shows primary/control pressure. The instructions that come with these testers do not supersede the DMC shop manual.

[deloryan]

No reason to change the accumulator unless you fail the "rest pressure test". Also you can remove the small line on the rear of the accumulator, plug the rubber hose, jump the RPM relay, and see if fuel is dribbling out of the accumulator where the hose used to go. If you are dripping, change the accumulator. If it's not dripping, it's fine, and your problem is somewhere else.

Where did you get the part # for the fuel pump? I have a new pump from a vendor but it's a Delphi.

Change the fuel filter, check the accumulator as I explained, and then re-test the primary pressure. Check the shop manual to see how to put the pressure tester in line with the CPR. It's easy to get it backwards in regards to the bypass that shows primary/control pressure. The instructions that come with these testers do not supersede the DMC shop manual.

I got the part # from another post on this forum. I believe the AC Delco MU1777 and the Delphi FG0089 are basically the same from what I can find online, but maybe something is different internally. I’ll check the accumulator as you suggested but might be the weekend before I get back to this. When I wasn’t getting the right pressure at the FD, I connected the gauge directly to the pump at the tank and was only seeing 5 bars there, so I’m not starting from a good pressure.

[FABombjoy]

I connected the gauge directly to the pump at the tank and was only seeing 5 bars there, so I’m not starting from a good pressure.
Was the return loop still in place? Fuel pressure is created by restriction so if you have a leakdown between primary and return the whole primary side would read low.

You should definitely perform the rest pressure check as mentioned by 82DMC12 as this will let you know if the accumulator is suspect, or if there is some other path that bypasses the PPR. Your module has been used in other K-jet DMCs successfully and should be OK.

[Ron]

^^^

You can also leak past fuel the accumulator seal to the return line which would present similarly. That is, higher pressure at pump than at fuel distributor. This is a failure mode not discussed much but happens with time.+1!

[deloryan]

Making some progress on the fuel issues. My primary pressure is still low, but about the same as at the pump, 4.5 bar. I replaced the accumulator, previously replaced the PPR o-rings, new fuel filter, and new-style fuel pump.

I pulled the injectors, one was barely a dribble, one sprayed a stream, and the rest were only misting out of half the cone. I'm getting new ones. Car starts fine, but won't accelerate. Idle is pretty shaky, too. Hoping most of my issues are now injector related. I still have the original fuel lines so will be replacing those at the same time.

I'm also cleaning up the CPR. My original post had a picture of the screen, and I pulled it out today, but had to drill a screw into it to remove it. What kind of material is this? It's like a copper cotton ball, but it's so crushed together that nothing was flowing through. I've ordered a rebuild kit, but now that there's no filter, can I put one of these filter baskets back in the hole or do I need to come up with something else? https://www.deloreango.com/us/fuel-distributor-injector-line-filter.html

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[Bitsyncmaster]

Making some progress on the fuel issues. My primary pressure is still low, but about the same as at the pump, 4.5 bar. I replaced the accumulator, previously replaced the PPR o-rings, new fuel filter, and new-style fuel pump.

I pulled the injectors, one was barely a dribble, one sprayed a stream, and the rest were only misting out of half the cone. I'm getting new ones. Car starts fine, but won't accelerate. Idle is pretty shaky, too. Hoping most of my issues are now injector related. I still have the original fuel lines so will be replacing those at the same time.

I'm also cleaning up the CPR. My original post had a picture of the screen, and I pulled it out today, but had to drill a screw into it to remove it. What kind of material is this? It's like a copper cotton ball, but it's so crushed together that nothing was flowing through. I've ordered a rebuild kit, but now that there's no filter, can I put one of these filter baskets back in the hole or do I need to come up with something else? https://www.deloreango.com/us/fuel-distributor-injector-line-filter.html

67169

That is normal for those crushed brass filters. I would check your control pressure without the filter installed. My guess is your WUR is bad if the control pressure is the same as your primary pressure.

[Ron]

Making some progress on the fuel issues. My primary pressure is still low, but about the same as at the pump, 4.5 bar. I replaced the accumulator, previously replaced the PPR o-rings, new fuel filter, and new-style fuel pump. I'd try powering the pump right off of a fully charged battery. And if the pressure, measured right at the pump, was still low, pull it. And if the pump intake was not restricted, I'd replace it.

[David T]

Typically the problem is not the fuel pressure, it is the injectors and vacuum leaks. Fuel pressure problems generally cause problems at high RPM's or under load. Have your injectors cleaned, very often injectors can be salvaged by a good cleaning using the injector tester/cleaner. If the injector seals are hard, replace them and use new copper washers.

[deloryan]

Thanks for the help everyone. I've got my fuel pressure dialed in and it is running great. Except with the A/C on. Then it really struggles to accelerate under load. Voltage at the jump start post is 13.6 with A/C off, 12.5 with A/C on. I'm thinking this must be voltage related. I've replaced the battery. Where should I be checking voltage? What should I investigate next?

Below are the other components I've replaced while getting the fuel issues sorted:

Fuel Pump - new style
Accumulator
Fuel Filter
Fuel Injectors
All new fuel hoses (replaced the 40 year old ones finally)
hot-start o-rings in PPR
Rebuilt CPR (rebuild wasn't nearly as difficult as I thought it would be)
Ran vacuum test and idle speed motor was leaking badly, so replaced that
New spark plugs
New battery

[MrChocky]

Thanks for the help everyone. I've got my fuel pressure dialed in and it is running great. Except with the A/C on. Then it really struggles to accelerate under load. Voltage at the jump start post is 13.6 with A/C off, 12.5 with A/C on. I'm thinking this must be voltage related. I've replaced the battery. Where should I be checking voltage? What should I investigate next?

Below are the other components I've replaced while getting the fuel issues sorted:

Fuel Pump - new style
Accumulator
Fuel Filter
Fuel Injectors
All new fuel hoses (replaced the 40 year old ones finally)
hot-start o-rings in PPR
Rebuilt CPR (rebuild wasn't nearly as difficult as I thought it would be)
Ran vacuum test and idle speed motor was leaking badly, so replaced that
New spark plugs
New battery

These voltages seem reasonable; but obvious things would be alternator and grounding. As an aside, DM's idle ECU does an "A/C bump" for idling with A/C, but that is not relevant to this.

[SupercoolBill]

I am curious, did you ever get this figured out?

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[deloryan]

I am curious, did you ever get this figured out?

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Yes, the final piece of the puzzle was the alternator. It was not making enough power at higher rpm. Once I replaced that it accelerates great again.

[SupercoolBill]

Yes, the final piece of the puzzle was the alternator. It was not making enough power at higher rpm. Once I replaced that it accelerates great again.Oh wow. So after all that it was a voltage issue?

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[deloryan]

Oh wow. So after all that it was a voltage issue?

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Well, there were definitely fuel issues too. Each thing I replaced made it run better, but the tricky one at the end was that the fuel pump was not getting enough voltage with the AC on. So while it would run great in some conditions, the alternator just couldn’t output enough under a heavy load. What tipped me off was brown goo on the bottom of the alternator. Tell tale sign the alternator regulator was melting out.

[SupercoolBill]

Well, there were definitely fuel issues too. Each thing I replaced made it run better, but the tricky one at the end was that the fuel pump was not getting enough voltage with the AC on. So while it would run great in some conditions, the alternator just couldn’t output enough under a heavy load. What tipped me off was brown goo on the bottom of the alternator. Tell tale sign the alternator regulator was melting out.Wow
Was that the original alternator? Motorola or whatever it was.

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