DAVECAD2.0

1992 LT1.
Trying to diagnose a rough idle. Found this. With fuel pressure gauge installed and switch on, the pressure only goes to 20 psi. As soon as the switch is turned off the pressure drops to zero.
When started the pressure goes to 41 psi. but idle is rough. Switch off and the pressure drops immediately to 0 psi. With engine running, and vacuum hose to fuel regulator disconnected, pressure climbs to 45 psi.
I can hear the fuel pump running with switch on. Chris Fix on a Youtube channel suggests it is most likely a check valve in the pump.
Your thoughts.
Thanks Dave.

Kevova

Key on should go to 43 psi or more. Check valve is part of pump, pulsator attaches between sender and pump. Either can cause fuel pressure to fall quickly. Pulsator can be replaced with a short section of hose. Check valve requires new pump. Restricted fuel filter could prevent the volume of fuel required to reach rail. Normally means tank needs cleaned.

aklim

You need help unless you are Reed Richards. Have someone turn the key and you crimp off the feed line a second you give the command. See what happens. If the pressure still drops, I think it is either your injectors leaking like hell or the regulator. He can observe it and it should spike up to 40 plus. If it doesn't, I'd look towards the tank

grandspt

Sounds like your fuel pump is bad. I don't recommend a Delphi replacement. I installed one and it took care of one issue (hi RPM fuel starvation) but caused another. The check valve seems to be bad because with key on it goes to 44 psi and loses pressure immediately.
Going to try a different pump (Deatchwerks DW200 kit from Summit).
Good luck! I have been finding that AC Delco and Delphi replacement parts for my 96 are crap including spark plugs, MAP sensors and ICM's.

Red86Z51

Take a look at the flowchart below and follow those steps. They will get you right to the root of your problem.





Let us know how you make out.

DAVECAD2.0

Red: All.
Thanks for the info. I've been using trouble tree on page 6E-3-A-33 of FSM. Don't have the where-with-all to install stop valves on the fuel lines as described on chart A-7.
So I checked for injector leak-down as instructed on page A-32 para. 4. With switch ON not a single drop from any injector. So, going after the pump. Don't know why but this thing eats fuel pumps. This will be the third one in 5 years. Been using Delphi and AC. Maybe try an Edlebrock. I only use Non-Ethanol fuel.
I'll let you know.
Thanks again, Dave

Red86Z51

Dave,

I've been using Bosch as my go-to for fuel pumps and have yet to have one fall out on me. I never go with the el-cheapo's. I've heard disparaging remarks about the Delphi's and most seem satisfied with the AC Delco units.

Will stay tuned...

Red--

aklim

Ethanol will kill Multecs but probably not the pump. Certainly not a Walbro one. I run E10 since I don't have Multecs and do not want to be chained to a few stores.

DAVECAD2.0

FYI All.
Pre-Ethanol cars or any engine, mowers, out boards, weed wackers, etc. Were not designed to use it. The problem is this. Ethanol degrades latex rubber and Neoprene seals, O-rings, gaskets, hoses,etc. The solution wast to switch to EPDM "rubber".

KyleF

Have you checked your voltage supply to the pump? Low Voltage can cause electrical components to cook.

DAVECAD2.0

13.4v

DAVECAD2.0

Solved the pressure problem. Not rough idle.
Eliminated the "Pulsator" after the pump. Tore it apart. This is not a pulsator. More precisely it is a pulse dampener. In the mechanical engineering world this is referred to as a "Snubber". A hydraulic snubber, or dampener is meant to even out, or dampen pressure spikes. But, they are typically used on diaphragm pumps. Not centrifugal pumps. Or, in systems with multiple outlets like a homes plumbing system where "water hammer" can occur. Our's is a single closed loop system with constant pressure. So, there's really no need for a snubber. Installed a 4" length of 1/4" fuel hose. Works great. So before replacing what could be a perfectly good pump, eliminate the "pulsator".

Red86Z51

Congrats and thanks for posting your fix!

I wish more folks would follow your example here and do the same. Too many times members ask for help, get it, then NEVER follow up when they've finally solved the problem.

Red--

DAVECAD2.0

Mama always said; "Be nice and share"
Still have that lumpy idle. Fires right up and has instant throttle response. Idles a bit lumpy with a bit of surging.
Checked the IAC and MAP sensors. Checked for vacuum leaks and replaced every line I could locate. Even the one from the cruise control to the brake pedal. What a PITA. Replaced all rubber fuel lines.
Meggered all the spark plug wires. Pulled, cleaned, and checked spark plug gaps. Cleaned the throttle body. Don't think its a fuel or spark issue.
Kindda outta things to check next. Old gas? Doesn't get driven that much. Don't use Ethanol fuel. But still...
Your thoughts.
TIA, Dave

aklim

Checked IAC and MAP, how?

Clean the TB, how? Did you take it out and remove the IAC housing and top plate to clean all passages?
Why bother to clean and gap the plugs? If they are out, just replace it with gapped plugs. Car came with copper plugs, IIRC. I wouldn't go with expensive plugs since all they do is extend the life, at best. At worst, after years of staying in place to get to 100k or whatever their life is, it gets stuck in the head.

Ethanol fuel that is fresh is better than pure gas that is stale

DAVECAD2.0

Checked IAC and MAP using the using the Youtube videos. Which mirror the FSM checks.
Removed and cleaned the TB and IAC. Years ago I did the TB coolant bypass. So the "well" under the top plate was clean.
Checked the plugs just to be methodical. No oil, just a thin coating of black carbon soot. If the're out why not check the gap?
I know all about plugs. Bought a brand new Silverado in 2004. 115K and never checked the plugs yet. Still runs great.
While I had the plugs out I did a compression check. 152 to 156 psi across all eight. Not bad.
Did a cylinder leak down test @ 100 psi. No detectable noise through the TB or exhaust pipes. Valves are tight. Pressure bleed off ~4 psi over ten minutes so the rings are still in good shape.
If you have the plugs out of an LT1 you do this stuff because there a huge PITA to remove and install. i.e. number 6 and 8. Not something you want to revisit.
It just feels like a vacuum leak or the intake air is not metering properly.
Going to rig up a smoke machine. Read some where that the TB shaft seal can leak.
Search goes on.
Thanks again Red. I'll keep you informed.

Dave.

Red86Z51

Staying tuned for the latest and greatest developments!

Red--

aklim

I'm not sure. Can you run through the thing a bit? How does it check the MAP? Run a little vacuum and a 5V thing to get voltage? How does it test the IAC? Resistance? ASSUMING it is right, what does the ECM think? Doesn't matter what you think or I think. I chased an idle problem setting it like the FSM said. It never worked and kept working. Found out it was a broken wire. Swore off that setting method. I use IAC counts to set it. That tells me if there is an air leak somewhere before I start chasing wild geese.

Didn't mention if you took the IAC housing off to get at the passages? Also did you reinstall it with fresh gaskets?

I had that issue too. Runs great at 75K. Gap was way larger and guess what? I picked up a couple mpg with only new plugs. Not a fan of "runs great" or "no problems". I don't bother with checking the gaps when I take the plugs out because the new ones I install will be gapped.

Again, forget what it feels like. See if the IAC counts reflect that. Adjust it till you get 30 counts. If it is more than 30 counts, you need to turn the screw in, rev and recheck. If it is less than 20-30 counts, turn the screw out, rev and recheck. THIS IS VERY IMPORTANT. If you cannot get it and it will be 0 counts even when you turn the screw in all the way, you probably have an air leak. Ask me how I know.

DAVECAD2.0

I meant to say I checked the TPS not the IAC. Just cleaned it. The TPS checked out SAT per the FSM procedure. Yes, I installed new gaskets on the TB, BBK 1583.
You check gaps on existing plugs for one simple reason. As the center electrode erodes down the gap increases.
A little more detail on how you checked the IAC please.
Thanks, Dave.

aklim

3rd paragraph of my last post. I use a scanner because I don't trust the FSM method. It assumes everything including the tach is within spec and the timing is on and there are no air leaks or any other issues. I tedt timing and fuel pressure first