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LT1 hesitation in RestoMod

2K views 8 replies 4 participants last post by  BALLSS 
#1 ·
To the Forum. I am at my wits end and asking for help.

I have a 1971 Cutlass Convertible with a 1995 LT1 swap I did about 4 years ago.

At the end of last summer something broke and has been causing a light throttle hesitation and when the car warms up you can feel a miss at a stop light some of the time.

The car runs fine at 75% to full throttle. This is a partial throttle issue and it happens all the time. It is not an intermittent issue.

There are not computer codes and I have replaced about everything I can find on the forums that have similar issues. I have even spent some money with a local expert who works on corvettes and had a mechanic in the shop that specializes in the LT1's.

Here is what I have changed with no change in issue. Most parts have been OEM except for the Wires, Mass Air, Intake air temp and Coolant temp sensors.

Wires, Optispark(2 of them, One AC Delco the other not), Spark Plugs, Coolant Sensor, Intake Air Sensor, MAP sensor, Throttle Position Sensor, O2 sensors, Mass Air Flow sensor, fuel filter, O rings on injectors and timing chain(it was pretty stretched out), trans fluid change but only what I could get out of the pan changing the filter.

Fuel Pressure is at spec at idle and when driving (taped fuel gauge to window while driving)

Took the fuel rail out and developed an injector cleaning system with some vacuum line and a can of injector cleaner. Pulsed each injector with a battery to verify spray pattern and leakage. Spray patterns looked good.

Changed the Intake Gasket

Changed every vacuum hose.

Cleaned throttle body and put new gaskets on.

Changed PCV

Have sprayed numerous times with carb cleaner looking for a vacuum leak with no luck. The corvette mechanic couldn't find any vacuum leaks either.

Took all grounds off and cleaned.

The wiring harness is aftermarket purpose built for resto mods and the 4L60 trans is part of the harness and used in the car. I have started to pin out the computer connectors to the sensors. I have pinned each injector, the Opti, Map, Mass Air Flow and O2's so far. No weird resistance from corrosion.

The car has no pollution control items such as EGR or Air injection, the car also has no Cats. Not required until 1972.

Exhaust is dual 2.5" with Dynomax mufflers, these are new as well in case the others were clogged.

The only issue I can see in the computer is the BLM is high. Runs in the 140's which from what I can read indicates the computer thinks the car is running lean. However, until this issue I never paid attention if this was part of the tune.

Any help or advice is greatly appreciated. I have a few free trials left on datamaster so if someone knows how to read these things I can take a run with it recording. I'm also willing to pay someone for time to help diagnose this issue so if anyone knows of an LT1 expert in the central New Jersey area please let me know. If not in New Jersey Skype works for me as well.

Warmest Regards,

1971 Olds.
 
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#2 ·
Welcome to the forum.

When the computer thinks mixture is lean, it's usually a vacuum leak somewhere.

Another idea is to mist plug wires with a little water. Then run at night and look for spark leakage.

Did rotor screws get locktite on them when opti installed?

Also, get a $70 pc based oscilloscope. Use it to monitor opti low & high resolution signals. Make sure they are the correct square wave, and don't drop out or change shape while driving.

Since you already have a data cable, go to the link I provide and download the free obd 1 scan program called "scan9495" It reads regular codes, as well as ABS & SIR codes. Does running sensor scans and will log data to a file for playback. It also tests engine & auto trans actuators. And it's free.
www.mediafire.com/?0elo4c935uc5s

Get the newest version from the v-8 folder.
 
#5 ·
COCOBOLO95,

I hope you can still help me. I finally got my oscilloscope and had some time to look at the optispark signals. I found no signal loss when the car was hesitating. I did notice in the scan 9495 program the knock was retarding the timing a few degrees when it was acting up. It also seems to be getting worse. I can now duplicate the symptom in the drive way with the foot on the break and hitting the gas.

Please note that when you go to full throttle all hesitation is gone and the car runs great. It is only at part throttle I have an issue.

I thought it could be the knock sensor and since this is a caprice motor it has a second one on the drivers side. The wiring kit I used only used the passenger side. I extended the wire and plugged in the driver knock sensor and it didn't help. Is it possible that the Knock module has gone bad?
 
#6 · (Edited)
While it's always possible that knock module is bad, it's not likely, however. If your getting spark retard at a certain rpm and load, you should invertigate if any knock is occurring at that range.

Putting the car on an obd 1 scanner that does real time sensor data would be helpful. If you do not have one, you can download the obd 1 scan program called "scan9495" at www.mediafire.com/?0elo4c935uc5s
Download the newest version in the v-8 folder.

You will need either a 16 pin or 12 pin aldl to usb cable to use the program. 95 cars came with the 16 pin aldl, but I don't know what you installed in your cutlass. Get at www.aldlcables.com

What you need to do is to record running sensor data under good running conditions, ie when not acting up.

Then make another data recording when you get car to act up. See which sensors are not within proper readings.

Scan9495 scans for regular codes, ABS & SIR codes. It does running sensor scans and will store data to a file for later playback. It also tests engine & auto trans actuators.
 
#8 ·
I have ran across this a few times on different vehicles that I've owned. A flutter or miss under a light throttle, ran great at wfo. Every time it was a cracked plug. I had just changed them every time it happened. As stated have someone foot brake it at night and look for the sparks. Those hair line cracks are hard to find.
 
#9 ·
OP

Just adding a few comments.

you noted the MAF is not GM if I read your first post right. Your PCM needs to be adjusted for whatever MAF you run (B-body, F-body or aftermarket Granatelli...and if it is a Granatelli ditch it for a GM one). Replacement MAF I have seen from chain auto part stores are reman GM ones although sold under whatever store brand or "cardone" label. Since you note no EGR I assume the PCM has been tuned to ignore that and has been tuned for whatever MAF you are using unless it is the one that came with the donor engine & PCM.

If you are running a MAF without a screen and also a K&N filter sometimes oil from those filters gets on the very thin sensor wire of the MAF and drive-ability problems occur. You can gently clean the wire with Q tip and electrical cleaner

The part throttle issue can be several things but LT1-xjs notes you could have plug arc. I chased this issue and also swapped out most of the parts you have before "discovering" the arc while under the car with it running and by chance saw it from underneath. So I encourage you to check every plug from every angle. If you are running those plug boot heat shields the arc can be between the boot and shield making it about impossible to see.

dialectic grease is your friend on plug wire boot ends
 
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