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**FIXED!!**DEFINITELY***Super HIGH idle after multiple parts replacement - TBI 1990 K1500 350engine
- Thread starter ZombieK5
- Start date
The truck is bone stock and does not have a fuel pressure gauge. Let me see if I can try and get one attached without having to modify the hard lines connected to the back of the tbi. Not that new parts are guaranteed to work but it is a brand new pump and sending it in the tank. The pump and sending unit are from Delphi.
I might be inclined to think fuel pressure too however when the map sensor is disconnected the truck runs like a champ. Fantastic throttle response no surging fantastic Idle.
For some reason the map sensor being connected throws the whole system into chaos.
I'll have the TBI back on the truck again today and I'm going to start testing vacuum values. My adapter cable for the ALDL will be in Saturday and I can hook it up to my laptop to see what kind of readings I get.
I'll go and pick up a fuel pressure tester that has a General Motors TBI adapter
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I borrowed AutoZones tester. Connected it at the frame to flex line section forward of the fuel pump. The fittings at the TBI are sketchy IMO, best to leave those alone. Just need to make sure their tester has the early GM fuel line fitting. It should, but they end up missing parts at times.
Fuel pressure shows just below 10.
Sequence in video (Note, MAP sensor is UNPLUGGED)
1. Key on engine off. Pressure rises to just under 10psi. I did not videotape it but if I leave the key on without starting the engine the pressure drops down to almost zero. Not sure if it is supposed to hold pressure prior to the engine starting.
2. Engine starts and pressure holds steady just below 10psi. The camera goes black for a few seconds so I can grab something to press down on the accelerator PEDAL.
3. Engine revs and pressure stays put at just below 10psi
Pump is a new Delphi that touts 12psi
4. 2nd video is engine off with pressure immediately dropping. Not sure if it is supposed to hold pressure for awhile.
Anyone? Thoughts?
View attachment Untitled.mp4
View attachment Untitled1.mp4
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gmbellew
Registered Member
fuel at 9 psi is bare min spec....
is the vac steady at idle, or jumpy needle?
is the vac steady at idle, or jumpy needle?
don't want vacuum on egr at idle. TBI run best at 12-14 psi should have that at idle.
gmbellew
Registered Member
there is vac on the EGR port at idle on the TBI, but the solenoid prevents it from reaching the valve.
Vac is steady at idle and reading was taken from the TBI
Vacuum reading was taken at the TBI and not the solenoid. I will check that later.
The manual says 9psi is bare minimum. Not sure if I need to replace the pump even though it is new.
Actual PSI was steady 10psi.
Key On = 10psi
Engine Running = 10psi
When engine was surging the PSI held steady at 10psi - NO fluctuations.
In theory, the pressure depends mostly on the pump and the regulator. If the problem was the fuel filter, you would expect the pressure to drop with load on the engine. Likewise, it's the pump and the regulator that hold pressure after the pump stops. I would suggest testing the pressure with the old pump if that swap was easy, but...
You could try rebuilding the pressure regulator (https://www.rockauto.com/en/catalog...+&+air,fuel+injection+pressure+regulator,6124) and there are even mods to make them adjustable. I've also heard of putting a washer above the spring to raise the pressure a little, but I haven't tried it. You could try pinching the return line to ensure the pressure goes really high, which kind of implicates the regulator.
There's also the chance that your pump isn't getting enough voltage, but generally that affects the top-end flow more than steady state pressure.
You could try rebuilding the pressure regulator (https://www.rockauto.com/en/catalog...+&+air,fuel+injection+pressure+regulator,6124) and there are even mods to make them adjustable. I've also heard of putting a washer above the spring to raise the pressure a little, but I haven't tried it. You could try pinching the return line to ensure the pressure goes really high, which kind of implicates the regulator.
There's also the chance that your pump isn't getting enough voltage, but generally that affects the top-end flow more than steady state pressure.
That all makes sense however, no one has been able to explain why it runs near 100% normal with the MAP Sensor unplugged. I do not believe that fuel pressure into the TBI and MAP sensor area connected in anyway. The fuel regulator seems to be a slave to whatever the internal spring allows.
So, WHY does the engine run fantastic with the MAP unplugged BUT runs like crap when it is plugged in?
I have another regulator I can swap in to see if that is the issue.
With map sensor unplugged the computer fall back on a fail safe table program, and Ignores the sensors. for fuel ratio and timing. May still use tps and trans vss, possibly knock sensor.
So, with that thinking....
1. Pretty sure MAP sensor is good unless I got TWO bad brand new GM sensors.
Issue can't be Vacuum. If MAP sensor disconnected, the vacuum leak would still be present.
2. EGR solinoid could be bad but I don't believe it activates until truck is fully warmed up. Truck surges when still cold.
3. TPS could be bad but doubt It would cause surging. It doesnt move when surging is happening. Also, I swapped the original TPS I had installed and the issue still happens. So... assuming TPS is OK
4. IAC seems to function fine. It doesnt seem that it's causing a problem. Could be totally wrong about that.
5. Spark controller. Haven't considered it yet.
6. Fuel pressure seems fine, even if in the lower pressure range. It does not fluctuate when surging which suggests surging is air intake related, vacuum or possibly another unknown.
I am going to swap in a new regulator. I have a spare that came with one of the kits I bought.
7. If it's something elee, I have no idea.
gmbellew
Registered Member
what's the condition of the O2 sensor?
Hopefully the data cable will show something in the sensor data or fuel trims to help nail down the problem.
you can also verify if it only does it in closed loop. that might help narrow it down some.
Hopefully the data cable will show something in the sensor data or fuel trims to help nail down the problem.
you can also verify if it only does it in closed loop. that might help narrow it down some.
It should hold pressure. I don't know as there is a spec, but GM programmed the ECM to only prime every ~15 seconds (via the ignition being cycled with the key) so they must have thought it should hold for at least that long. I had a split hose in the tank (from ethanol, known problem with old hose that wasn't rated for it) and it would build pressure and immediately drop. New hose shouldn't have that issue. But if so, problem normally lessens on a full tank.
IMO with that low of pressure when running, hard starts, and running smooth on fuel tables that are richer than what it normally would be (MAP disconnected) it indicates a potential fuel issue.
It idles better cold too, yes?
IMO with that low of pressure when running, hard starts, and running smooth on fuel tables that are richer than what it normally would be (MAP disconnected) it indicates a potential fuel issue.
It idles better cold too, yes?
gmbellew
Registered Member
TBI doesn't hold pressure. it bleeds off quickly
Thats different than what GM says.
"For a faulty in-tank fuel pump check valve, which would allow the fuel in the lines to
drain back to the tank after the engine is stopped. To check for this condition:
1. Ignition "OFF.”
2. Disconnect fuel line at the filter. See "Fuel Control System,” Section "4.”
3. Remove the tank filler cap.
4. Connect a radiator test pump to the fuel line and apply 103 kPa (15 psi) pressure. If
the pressure will hold for 60 seconds, the check valve is OK."
I wouldn't discount the manual being incorrect, but if the system does not hold pressure, every TBI truck would have a hard start if key on/engine on doesn't happen in two seconds without cycling the key again. I don't feel that is the case, but I don't drive a TBI truck either.
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gmbellew
Registered Member
there may be a check valve in the tank on the pump, to keep some fuel in the lines, i don't know for sure. but it doesnt keep pressure at the fuel filter or TBI for long. the system primes for 2 second when you turn the key to ON. if you don't start it, the pressure will bleed off as measured at the TBI or just after the fuel filter at the flex line. that doesn't mean there isn't fuel in the lines, just means it isn't pressurized. That may be why the test procedure doesn't just use fuel pressure measurement, but actually pressurizes the system.
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