91 c1500 stutter on acceleration

[ChevyTech]

You're welcome Mark, and thank you for the kind words.

The TBI system is the easiest to help people work on. It is a tough durable system too.

[badass66ss]

Update: Last saturday i decided to drive the truck out to my uncle's for further electrical testing and left the O2 sensor unplugged. It finally through a cel about 10 miles down the freeway. The O2 connector was in bad shape and it had 170k miles on it so i replaced it. I checked all of the operating voltages with the truck at temperature and everything seems to be fine. This saturday we are going to begin looking for mechanical problems: fuel pump, fuel return line, pull the valve covers and check the valve adjustments, check vacuum and make sure intake isn't leaking etc. I will update on all findings. Thanks again for everyone's help!

[mountie]

Originally Posted by badass66ss

Update: Last saturday i decided to drive the truck out to my uncle's for further electrical testing and left the O2 sensor unplugged. It finally through a cel about 10 miles down the freeway. The O2 connector was in bad shape and it had 170k miles on it so i replaced it. I checked all of the operating voltages with the truck at temperature and everything seems to be fine. This saturday we are going to begin looking for mechanical problems: fuel pump, fuel return line, pull the valve covers and check the valve adjustments, check vacuum and make sure intake isn't leaking etc. I will update on all findings. Thanks again for everyone's help!

This thought may be a bit old / elementary, but before undoing stuff on the top of the engine (vacuum leaks)...... get some carb cleaner and spray it all over the engine listen for an idle change. That's how I found a cracked TBI gasket.

[badass66ss]

mountie, that is an old but valid check ha ha, and it works! Unfortunately, it didn't produce any results in this case.

[badass66ss]

Alright guys, we had some time to work on the truck this weekend. We pulled the bed and checked the fuel pump. The strainer was full of black stuff so we changed the pump and strainer, blew out the fuel lines, and buttoned everything back up. We then decided to check the cat. When we pulled it off it was about 60% plugged, we replaced it with a high flow 3" cat. Finally, we decided to pull the valve covers and check valve adjustment. Adjustment looked good so no change here. Took her for a test drive, no change... The only item that has not been replaced is the EGR, which I plan to test this week. Also, I was on empty yesterday after about a hour of driving around and filled up with Chevron 91. The truck seemed to run better, but I had a lot of "honey do" items in the back and could not get on it very hard. I am going to drive it tonight and see if there is a change or not. This leads me to think that the problem is ignition/timing related. I am currently 5 degrees advanced, maybe the wonderful low octane fuel that they shove down our throats just can't take it...

[mountie]

Originally Posted by badass66ss

Alright guys, we had some time to work on the truck this weekend. We pulled the bed and checked the fuel pump. The strainer was full of black stuff so we changed the pump and strainer, blew out the fuel lines, and buttoned everything back up. We then decided to check the cat. When we pulled it off it was about 60% plugged, we replaced it with a high flow 3" cat. Finally, we decided to pull the valve covers and check valve adjustment. Adjustment looked good so no change here. Took her for a test drive, no change... The only item that has not been replaced is the EGR, which I plan to test this week. Also, I was on empty yesterday after about a hour of driving around and filled up with Chevron 91. The truck seemed to run better, but I had a lot of "honey do" items in the back and could not get on it very hard. I am going to drive it tonight and see if there is a change or not. This leads me to think that the problem is ignition/timing related. I am currently 5 degrees advanced, maybe the wonderful low octane fuel that they shove down our throats just can't take it...

Generally, if you were not getting engine ping with regular gas, regular is the best choice. Regular is also the cleanest gas at the pump due to the storage tank getting cycled more than the other tanks at the station.

My truck's OEM timing mark is "0". If you are 5 degrees off, that could be an issue.

[ChevyTech]

Badass66ss have you tested the compression on all cylinders of this engine?
If not it would be a good idea.

About the fuel you use:
The TBI 350s have a 9 to 1 compression ratio.
The TBI 305s have smaller combustion chamber heads with a slightly higher 9.3 to 1 compression ratio.

I have found the 305s will run best on the 89 octane rather then the 87 octane ethanol blend they sell here. I have tried 91 octane in my 305 and found it ran best on 89 octane.

The electronic spark system will retard the timing if it detects pinging. If you advance the timing from spec the spark control will remove the excessive timing when the system is warmed up and running in closed loop operation.

When I run 87 octane in my 305 during the summer I can hear an occasional ping and the system backs down the timing to the point I can notice a lack of performance. I is much more pleasurable to drive with 89 octane and I hear no pinging. The performance difference is noticeable.

[badass66ss]

Ok guys it has been quite some time since I have had time to post any updates! So after all of this time we decided to slow down and diagnose this problem step by step. What we found: lifter with a bad check valve and the timing chain was stretched out over 1/2". Once we finished replacing the lifters and timing chain, we set the timing and found that the truck was running 99% better. It still had a slight stutter and actually backfired through the exhaust. We then unplugged The timing connector, set the timing 12 degrees advanced, and took it for a test drive. It ran like a bat out of hell!!! What would cause it to run rough with the connector plugged in?? Thanks again for your help!
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[ChevLoRay]

Read thru all the posts and crap! It looks like every now and then, the problem can actually be something mechanical, and not a sensor or computer gizmo. ChevyTech has been a great resource in the past, and he's still at it.

Whoodathunk a lifter with an issue? I mean, that's old school, dude. A stretched timing chain? Don't these engine run 200K miles without maintenance? Honestly, it's a dose of reality to read thru your problem and solutions.

Thanks for the feedback on the cure.

[badass66ss]

ChevLoRay, it seems as these trucks are getting older with more miles, mechanical issues must be taken into consideration. Unfortunately, poor maintenance by the previous owner is most likely the cause of all of the issues we have found. Hopefully with some help we will find the final problem with the timing control. ChevyTech has definietly been a lot of help to users of this forum and we all appreciate it!!

[ChevyTech]

Thanks for vote of confidence and kind words guys.

The timing control (Electronic spark timing) you ask about could definitely be causing problems.

That fact that it runs better with the timing that far over advanced is a sign of a spark control system retarding the timing.

Knocks, rattles, ticking, clanking, what ever you have making noise, on, in, or bolted to the engine, should be considered as having the potential to cause the timing to get retarded by the system.

The Knock sensor contains a piezoelectric crystal which causes a voltage to be produced. Loose components on or in the engine can cause the knock sensor to create a voltage and the system may interpret this as pinging. The system will back off the timing to try to stop what it thinks is pinging.

Loose torque converter bolts can cause a false knock signal to be detected.

If there are very excessive knock counts the system should set a code 43.

[badass66ss]

It is not throwing any cel at all. everything on the motor is tight. We have not had the tranny out so I'm not sure about the Torque Converter bolts, but the tranny was replaced some time before I bought the truck. The knock sensor has been replaced as noted much earlier in this discussion. Other than loose components causing a "knock", is there anything else that could cause the timing to retard like this? Do you think the ECM could be bad again? I read somewhere that the battery should be disconnected each time you plug in/unplug the bypass wire. Is this correct? Could this short out the ECM? Thanks again for your help!!!

[ChevyTech]

Originally Posted by badass66ss

It is not throwing any cel at all.

It would be a good idea to check it for codes anyway.

Originally Posted by badass66ss

….Other than loose components causing a "knock", is there anything else that could cause the timing to retard like this?

Underhood wiring routing of the harness by and to the knock sensor can cause false knock counts.

Originally Posted by badass66ss

Do you think the ECM could be bad again?

I think that is a low probability.

Originally Posted by badass66ss

I read somewhere that the battery should be disconnected each time you plug in/unplug the bypass wire. Is this correct?

When you unplug the set timing wire a trouble code 42 gets set. Disconnecting the battery is one way of clearing the code. I would just pull the ECM fuses to clear the code. Pulling fuses has less risk to the ECM then disconnecting the battery.

Originally Posted by badass66ss

Could this short out the ECM? Thanks again for your help!!!

You are welcome.
Disconnecting the battery does have a small risk of damaging the ECM. If you forget the key on and disconnect the battery the risk is significantly higher of doing ECM damage.

I read the thread again and you have covered most things that can be a problem. Looking at the data with a scan tool would be a good idea but most of the time it takes somebody with experience to recognize the problem with the readings.

Some people have used WinALDL and posted there results on the message boards and I have been able to identify problems.

Online it can be very hard to find a difficult problem. If you were my next door neighbor it would make it much easier.

Consider double checking some of the things you have ruled out along the way.

[Mark S]

As far as the timing (12 deg. advance) is it possible (it is) that the harmonic balancer has moved, skewing the the reading?
Mark