New to site - 96' L31 problems.

[Wigglesworth]

Newbie here. Lurked for a while, decided to join and possibly get some insight to my truck problems.

I've had my 96' cab and chassis 3500 for a couple years now, and it's been a constant fight to keep it running right. It's had a hard life, but mechanicly seems sound. Truck has 4:10 gears and a 4L80 trans.

Here's a short list of what's been done since I owned it....

Upgraded MPFI spider
Replaced exhaust from down pipes back. No cats (some bozo cut em off already) dual super 44 flowmasters and turn downs.
Plugs, wires, cap and rotor
Fuel filter
Air filter
Cam retard checked with tech 2 scanner - spot on
Fuel pressure is 60psi key on, engine not running
Running is 55psi, spikes to 60 when accelerating

Compression checks at 170psi +/- 5psi on all 8 cylinders.

Verified every sensor and every ground I can find. Verified injector signal via node lights, etc, etc...

Ok, now on to the issues....

Cruising speed, truck will stutter. Not like shake, but just like you turned the key off and on real fast. Always recovers. Mostly happens on deceleration after creating a hill.

Truck seems to have great power, until I hook up to a trailer. Then it feels like your dragging a parachute behind us. The trailer is a standard 16' utility with 2 mowers. 3000lbs tops. All bearings are good. I would think the truck wouldn't even know it was back there.

But here's the kicker, it doesn't do it all the time. It's intermittent. Some days it runs like its on rails, some days it runs like crap.

I'm leaning on replacing the distributor or fuel pump, but I hate to throw money at it.


Any help guys?

Thanks in advance.

 

[Wigglesworth]

Forgot to mention, distributer gear looks great, and truck throws no CEL codes.

 

[skylark]

When I think of odd issues like that I am suspicious of an ignition module but in the case of the vortec my number 1 is the crank sensor and its wiring.

 

[Wigglesworth]

When I think of odd issues like that I am suspicious of an ignition module but in the case of the vortec my number 1 is the crank sensor and its wiring.

I swapped crank sensors with a parts motor with no change. I'll verify the wiring tomorrow morning...

 

[meterman]

I would change ecm and fuel gauge sender mine did that sender was pulling to many amps shut off and come back on till burned up ecm

 

[Wigglesworth]

Ok,

Hooked up to a live data scanner, and bank 2 was showing some odd readings. Crawled under and found O2 sensor was almost unplugged. Reseated and fired up truck and it ran extremely rough for a couple minutes and leveled out. LTFT and STFT are staying within +/- 5 and all seems well, however truck still has no power.

Then I noticed the spark advance was running at -25 at idle and crept up to - 5 or so under load. I unplugged knock sensor with no change.

For some reason my timing advance seems backwards??

Why is timing so retarded?

 

[Wigglesworth]

I would change ecm and fuel gauge sender mine did that sender was pulling to many amps shut off and come back on till burned up ecm

This truck has definite fuel gauge problems. Was a dual tank truck, then someone tried to eliminate the rear tank, and installed a single tank gauge cluster and tried to bypass balance module.

It's a mess under there....

 

[Mrich0908]

Whats fuel pressure to the rail? How quick is fuel leak down? What spark plugs are in it? You do know you have to set the distributor with a scan tool only on these motors . If off it could not throw a code but give problems.
Next look for vacuum leaks . Hows the brake booster? Make sure the booster one way valve and pcv working properly . Mend any broken vacuum hose. Did you do a lower intake manifold gaskets on it? These motors are notorious for it? Did you seal the spider and upper intake properly . Could cause a leak.
Whats the temp say from the ect say on the scanner ? Not your dash the ECT are known to malfunction and cause problems.
Pull your plugs what do they look like? How are your cats.
Is your EGR working properly? Gasket sealing properly? Clean it anyways.
Distributor caps go bad and arc on the inside.There is also a screen in the dizzy that clogs up creating moisture.
An holes in your exhaust to give false o2 readings.
These are all issues that could cause your problem that I know of.

 

[Wigglesworth]

Whats fuel pressure to the rail? How quick is fuel leak down? What spark plugs are in it? You do know you have to set the distributor with a scan tool only on these motors . If off it could not throw a code but give problems.
Next look for vacuum leaks . Hows the brake booster? Make sure the booster one way valve and pcv working properly . Mend any broken vacuum hose. Did you do a lower intake manifold gaskets on it? These motors are notorious for it? Did you seal the spider and upper intake properly . Could cause a leak.
Whats the temp say from the ect say on the scanner ? Not your dash the ECT are known to malfunction and cause problems.
Pull your plugs what do they look like? How are your cats.
Is your EGR working properly? Gasket sealing properly? Clean it anyways.
Distributor caps go bad and arc on the inside.There is also a screen in the dizzy that clogs up creating moisture.
An holes in your exhaust to give false o2 readings.
These are all issues that could cause your problem that I know of.

Original post answers a lot of these questions.

Has hydroboost brakes, so no booster.

I asked my mechanic (works at dealer btw) about lower intake, and he said that if it wasn't loosing coolant by now, it's prob had them done already. He installed the upgraded spider. And set the cam retard offset with a tech11 scanner.

Shows 194 degrees on scanner.

Plugs were new AC Delco approx 3k miles ago. Cap and rotor were replaced with Borg Warner at same time.

No cats. Previous owner hacked em off. I recently had the exhaust donuts replaced, and installed new exhaust from down pipes back. 2.5" with dual super 44 flowmasters. No leaks.

EGR has been cleaned and cycles properly when key is cycled.

As for vacuum leaks, I've not done extensive testing, other than spray carb clean around intake, with no change.

 

[Mrich0908]

Original post answers a lot of these questions.

Has hydroboost brakes, so no booster.

I asked my mechanic (works at dealer btw) about lower intake, and he said that if it wasn't loosing coolant by now, it's prob had them done already. He installed the upgraded spider. And set the cam retard offset with a tech11 scanner.

Shows 194 degrees on scanner.

Plugs were new AC Delco approx 3k miles ago. Cap and rotor were replaced with Borg Warner at same time.

No cats. Previous owner hacked em off. I recently had the exhaust donuts replaced, and installed new exhaust from down pipes back. 2.5" with dual super 44 flowmasters. No leaks.

EGR has been cleaned and cycles properly when key is cycled.

As for vacuum leaks, I've not done extensive testing, other than spray carb clean around intake, with no change.

Im sorry I probably didnt read your whole first post . I have bad A.d.d. I cant pay attention for nothing.
The vacuum is probably good . You could rent a gauge to make sure. You would know if you had a intake leak . It will drink water .
How about Cam and crank sensor. Sometimes on there way out they act funny but do not throw codes. Read about the Crank sensor.
Test your knock sensor . I believe we have two . Stroker guys have this problem from time to time. There is one on the block and one in the ecu from what Ive read. Run a live scan see if its pulling timing when accelerating.
Also there s a Park neutral safety switch when on its way out could create a problem like this. Its designed not to let the truck run unless its in a certain position.
I dont post here much , 33 post in 6 years . I do feel your pain . I understand how hard it is to find a problem thats not obvious. I just went threw the same thing. Mine turned out to be a slew of problems from the prior owner not maintaining the vehicle properly.
I had a fuel pressure regulator leak , bad fuel pump , intake manifold was leaking , bad ect , worn brake booster intake manifold Oring. After all that I sill had a misfire at idle. The a CKP sensor code popped up. Changed it still had a misfire. CKP sensor code poped up . Cleaned it . Misfire at idle went away.
After all that though , I gas , coolant washed my engine . I wore my main bearings out compression is leaking past the rings also.
So It may be multiple things. The best answer I could give you is . I used a program called scan XL on my laptop coupled with a 15$ elm27 dongle. You could drive around shart trips and see what the ECU does recording it to watch later. If you know where to look its free. It was better than most DIY cheaper scanners you can find. With a scanner and multi meter start probing sensors see whats doing what but be prepared for it to be more than one thing sometimes. I know you said you have tested sensors but sounds like you have an intermediate problem. Wiggle wires while testing.
If I could help you in any way PM me .
Im sorry I couldnt help you more Goodluck