Low/fluctuating Idle and No Start

[Antichrist]

Problem with idle, slow start and occassional no start.
1995 Chevy Tahoe. 5.7 TBI. Automatic. 136k miles.

In park: Idles ok at around 900 RPM, but a little rough. Giving it gas up to 2k RPM in park, engine shakes when decellerating.
In gear: Accelerating/High RPMS the truck runs like it was brand new. When I stop at a light, RPMs drop to 500, Idles rough, and RPMs fluctuate intermittently, dropping below 500 RPM, then correct themselves and come back up to 500 or so.

Also had a stalling problem, replaced MAP, and the truck no longer stalls when I come to a stop light. When it DID stall, It would start right back up with no issues.
It still takes 5-8 seconds of cranking before the truck will start.
I have had the truck not start for me twice. One time it took about 10 minutes of repeated cranks before it started finally. The second time I gave up and got a ride.

So far: Dist cap, plugs, wires, all check out ok. All terminals inside dist cap were cleaned. Ignition coil terminal cleaned. Otherwise No burns, cracks, etc.. wires/plugs were just done about 5k miles ago.

EGR: With engine off, diaphragm movement has resistance. Engine on, more resistance, when I push diaphragm, engine runs rough.

IAC valve has been replaced with a new one, no change in idle. I also tested a used IAC that I had, and no change. With truck running when I unplug IAC, there is no change in Idle.

TPS has been back probed, showing 4.55v with throttle plate closed, as I open throttle plate, voltage slowly decreases down to about 1.25V

Coolant temp sensor (sender?) has been unplugged, and truck runs the same. This is the unit between #1 and #3, in cyl head.

All fuses have been checked, and are fine. I used sandpaper to clean all terminals of fuses in engine block fuse block as there was a slight varnish on the blades. After doing this the truck started PERFECTLY, with a 1 second crank. It only started like this ONCE. After that, it was back to 5-8 cranks before it would start. I'm not sure if this was coincidence, or not.

When driving with a cold engine (ran for about 1 minute before driving) on a 50 degree F day: Acceleration is sluggish, if I give it too much gas too fast, it acts like it wants to stall. After the truck warms up, RPM idles at around 500, once I get over 1k, I can give it as much gas as I want, there's no stalling, no misfires, no pinging, no knocks, etc...

When RPMS drop to 1k-1500RPM when holding steady speed, the roughness comes back.

When I'm at a stoplight, and the RPMS start to drop below 500 and the truck starts running really rough, I can shift into Neutral, and the idle immediately returns to normal.

Everyone keeps telling me it's the fuel pump. I seriously don't think the fuel pump is an issue.

I bench tested the fuel pump relay, and it operates fine.

I listened to the relay with a stethoscope when installed, and had my girlfriend turn the key, and it operates fine.

I could never hear the fuel pump inside the truck, so I listened with the stethoscope underneath the truck, and the fuel pump runs fine.I had my girlfriend cycle the key on/off several times for the relay, and the pump, and I have no change in performance, sound, duration, etc....

Please help.. this is driving me nuts.

 

[SunlitComet]

Did you actually get fuel pump pressure while installed I know it is a pain on tbi but it is important. any possibility a vacuum line is misrouted or cracked. and finally if you have timing light aim it down the TB and look at the fuel spray pattern.

 

[Antichrist]

I did not test fuel pump pressure yet. I don't have a guage on hand, and nobody was able to tell me where to test the pressure, so I started ruling out other possibilities first.

I did in fact check injector pulses, at idle, and while opening the throttle, and the response looks fine. This is my first time having to actually test injectors, but as far as I can tell, they're functioning 100%

Yesterday I found information online that could help diagnose a possible vacuum leak, since I've seen similar symptoms before on other cars, and it could very well cause idle problems.

This is the exact procedure in order:

Key to on, engine off
Short pins on ALDL
Wait for codes to run
unplug IAC valve harness
then remove ALDL jumper
turn key to off
wait 15 seconds
start truck

If It idles high, there is a possible vacuum leak. My truck started around 1200, and climbed to about 1700.

So I checked with some carb cleaner. Checked every vacuum fitting I could find. I gave hoses and fittings a quick visual inspection. Checked along base of Intake, and I didn't get any change in idle. All vacuum lines are routed properly according to the tag on my fan shroud.

HOWEVER... While running codes, I received 2 ..

DTC 22: No 2nd gear
DTC 54: Powertrain Control Module (PCM) computer circuit problem - analog to digital (A/D) convertor error.

I took the truck out for a drive, and I do in fact have 2nd gear. 1, 2, 3, 4, all shift flawlessly.
Is the PCM the same as the ECM? Is there any chance a bad sensor could cause this code?
Should I further test the PCM?

 

[SunlitComet]

I am sorry to say but my coding reads something completely different.


Powertrain Control Module - PCM

Diagnostic Trouble Code 22

TP Sensor Circuit



CIRCUIT DESCRIPTION
The Throttle Position (TP) sensor provides a voltage signal that changes relative to the throttle blade. Signal voltage will vary from about 0.5 volt at idle to about 4.0 volts at Wide Open Throttle (WOT) .

The TP signal is one of the most important inputs used by the PCM for fuel control and for most of the PCM control outputs.

DTC 22 WILL SET WHEN

• Engine running.
• TP signal voltage is less than 0.15 volts for more than 4 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
The Malfunction Indicator Lamp (MIL) will illuminate, transmission TCC will not apply, high transmission line pressure, harsh shifts, fixed shift points and no fourth gear in hot mode.

DTC 22 WILL CLEAR WHEN
Conditions for fault are no longer present and Tech 1 "clear DTCs" function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and battery feed voltage is removed for 10 seconds.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Verifies TP signal display.
2. Simulates DTC 21: (high voltage) If the PCM recognizes the high signal voltage the PCM and wiring are OK.
3. This step determines a poor connection or faulty TP sensor.
4. This simulates a high signal voltage to check for an open in TP sensor signal circuit.

DIAGNOSTIC AIDS
A Tech 1 scan tool reads throttle position in volts and should read about 0.5 to 1.25 volts with throttle closed and ignition "ON" or at idle. Voltage should increase at a steady rate as throttle is moved toward WOT.

An open or short to ground in CKT 416 or 417 will result in a DTC 22.

Refer to Diagnosis By Symptom / Intermittents See: Symptom Related Diagnostic Procedures\Intermittent Condition

Scan TP sensor while depressing accelerator pedal with engine stopped and ignition "ON." Display should vary from below 1.25 volts (1250 mV) when throttle was closed, to over 4.0 volts (4000 mV) when throttle is held at WOT position.


Powertrain Control Module - PCM

Diagnostic Trouble Code 54

Fuel Pump Circuit



CIRCUIT DESCRIPTION
The status of the fuel pump CKT 120 is monitored by the PCM at terminal "B12" and is used to compensate fuel delivery based on system voltage. This signal is also used to store a DTC if the fuel pump relay is defective or fuel pump voltage is lost while the engine is running. There should be about 12 volts on CKT 120 for at least 2 seconds after the ignition is turned "ON," or any time reference pulses are being received by the PCM.

DTC 54 WILL SET WHEN
The voltage at terminal "B12" is less than 2 volts for 1.5 seconds since the last reference pulse was received. This DTC is designed to detect a faulty relay, causing extended crank time, and the DTC will help the diagnosis of an engine that "Cranks But Will Not Run."

ACTION TAKEN (PCM WILL DEFAULT TO)
The PCM will enable the MIL.

DTC 54 WILL CLEAR WHEN
Conditions for fault are no longer present and Tech 1 "clear DTCs" function is used or 50 ignition switch keycycles have passed with no further faults or ignition switch is turned "OFF" and battery feed voltage is removed for 10 seconds.

DIAGNOSTIC AIDS
Refer to Diagnosis By Symptom / Intermittents See: Symptom Related Diagnostic Procedures\Intermittent Condition

---------- Post added at 06:31 AM ---------- Previous post was at 06:27 AM ----------

Fob tbi unit you basically have to tee in a hookup just past the filter to install a pressure gauge.

But look over the stuff I showed above and make sure were are on same page in case I am missing something.

 

[Antichrist]

I was just coming to post a correction, I realized I ended up looking at Saturn transmission codes in my manual....

This code crap is confusing at times.

But Yeah, loooking in the CORRECT section on my code book, you're absolutely right. Code 22:
TPS Low voltage during idle (which I have issues at idle)
OR
Fuel cutoff relay circuit open or shorted to ground.

Code 54:
Low Fuel pump voltage
OR
Fuel pump relay
OR
EGR solenoid #2 failure
OR
Quad-Driver module (QDM) output failure
OR
Mixture Control (M/C) solenoid - circuit voltage too high.

Thank you so much for all the additional information. I don't have anything in front of me right now other than a code book with 2 sections. I would have assumed GM and Saturn used the same code system. In any event.. Thank you again. Now I have a little work ahead of me to narrow down the cause of my problem.

further testing of the TPS:

.23 V idle. 4.0 V wide open. Tapped TPS at idle and wide open, no fluctuations.
Moving on to look into fuel cutoff relay circuit.

 

[Antichrist]

Here is where I am:

I first cleared all codes by removing ECM fuses.

For DTC 22. For all TPS sensor testing, all test results are pointing to a faulty sensor, or faulty sensor connection.

I have not looked into the fuel cutoff relay circuit (open circuit/shorted to ground suggested) yet, as I'm having trouble actually finding the circuit.

For DTC 54 I was able to swap out the fuel pump relay for a known working unit, the problem was still there. Slow start, and stumble at idle.

I confirmed condition and connections of all emission lines, everything is correct, and in good working order.

Removed and cleaned EGR. Light carbon deposits, nothing severe. EGR ports in manifold are clear, also with light carbon deposits, but nothing severe. Manually tested EGR by hand (No vacuum pump in my posession) and no problems with airflow from vacuum tube.

Truck still runs the same, slow starts, and stumble at idle. While driving the stumble in gear, at idle, still disappears when I shift into neutral or park.

I thought I could show this better by recording a video. I started recording as soon as the RPM fluctuations start. The truck is in gear, and my foot is on the brake the whole time.

You can hear the idle dropping, you can see RPM needle up and down, And at one point you can even hear the truck starting to vibrate hard before it corrects itself, and jumps the RPMs. After that levels off, I put it in park, RPMs surge, then level off.

Base idle is right about 550 RPM right now, while in gear.

Direct link: http://www.youtube.com/watch?v=4cJyu2VtNog

I have not yet had any codes return since I cleared them a couple hours ago, but the problem remains.

 

[Phone Man]

Before pulling the fuel pump, replace the fuel filter. Try some Chevron Techtron injector cleaner too.

 

[Antichrist]

Other than quick testing to make sure the pump relay, and pump both operate, I haven't explored them yet. I also ran some gumout injector cleaner through the truck with no changes. It seems like every time I look, I find another problem, or possible cause to my problems..

I have put about 350-400 miles on the truck since its last oil change. I was about 1 quart low today. I also notice that my oil pressure guage didn't seem to be working properly. Off, it's at 0. On startup, I'm reading 60psi. about 55 psi in gear, then it slowly drops to 50 psi. Shifting back to park brings the psi up to 60 ish.

Seeing the oil pressure is what prompted me to check the oil. which as I said, was about 1qt low, after changing the oil about 350-400 miles ago.

Oil filter adapter gasket, and O ring were replaced at the same time.

I decided to look more into the possibility of a faulty oil press sensor/sender, and located the unit on the rear of the block near the distributor. It does look like it's leaking. But it doesn't look like 1qt in 400 miles, but I could be wrong. There is also no sign of oil in the coolant, and vice versa.

Anyways, while looking at the oil sensor, I noticed a tube, looks to be about 3/8 or 5/16, laying on top of the bellhousing. I felt around and can't find anything this might go to.

Ideas? Here's a picture, click for a larger image:

I've also explored the possibility of a stuck tstat, which was found to be the cause for identical problems on another truck I read about online, But my truck doesn't overheat, the coolant guage works, and the heater inside works absolutely great.

 

[SunlitComet]

You are gonna have to find the other end of that hose. It does not look like a vent for a tranny or t-case. I also don't believe there should be a hose there any way.

 

[Antichrist]

You are gonna have to find the other end of that hose. It does not look like a vent for a tranny or t-case. I also don't believe there should be a hose there any way.

That's one of the first things on the agenda tomorrow. I couldn't climb underneath it today since it's been pouring rain on and off all day.

I'll post up what I find, and I plan swapping in a known working TPS just to completely eliminate the TPS from the equation, or possibly find out that it has an intermittent problem.

Thank you all again for all the help and suggestions. It's saving me a lot of time going through dead end google searches.