TPIS Modification Help

[72Slydog]

Hello everyone! Need advise from the Tuned Port Injection experts on the Forum. Experiencing rough idle and loss of power condition. I have a 1972 Corvette coupe with a 1996 ZZ3 GM Crate motor installed with the following options;

MSD 6AL Ignition, MSD Master Blaster 2 coil, MSD 8571 Pro Billet distributor, 1990 Tuned Port Injection system, Painless fuel injection wiring harness. Car had been running great but started noticing performance problems, like a fouled plug or bad plug wire. Getting diagnostic codes 42,43,44. Engine acted like it was only running on seven cylinders. Very rough idle. Putt Putting sound from left side exhaust. Goes away at higher rpms. Vacuum reading at 15psi.

Checked all vacuum fittings, hoses for leaks, installed new MSD 8mm plug wires, rotor, cap, new AC FR5LS plugs, MAP sensor, oxygen sensor, knock sensor. Codes 43 and 44 went away but still getting code 42. Chevy dealer says that I'm using the wrong distributor and this is causing the problem because ECM is not receiving signal from distributor for spark control. I can't use the original HEI distributor that came with the engine because it lacks the mechanical tach drive feature. Does anyone have any suggestions on eliminating Code 42. What else should I check to pinpoint the performance problem? Also removed and cleaned the throttle body and IAC. Reset TPS to .54 V. Injectors ohmed out at 17.4 ohms on each one. Still need to verify fuel pressure and run compression check.

Any help would be greatly appreciated.

[patsnitrovette]

well you pretty much covered your basics,has it run good anytime or did it just start running bad,also you really should have the hei dist. hooked up,so your 4 wire est plug is disconnected then right?

[BillWelter]

This sounds like a common engine problem, not specifically a Tuned Port problem. Let the car idle and pull the spark plug wires, one at a time, to see which cylinder it is. If the idle gets worse when you pull a wire, that's not the bad cylinder. If the idle doesn't change, that's the one. You may have to get someone to hold the engine at a faster idle so the IAC won't compensate. When you find the bad cylinder, check the spark plug and wire, If possible, switch them with the set from another cylinder. If the problems still stays with the cylinder, pull the valve cover and check the valve adjustments. If the exhaust valve is too tight, it will cause what you are describing. A cam with a flat lobe will do it too.

[72Slydog]

Hello Patsnitrovete

Four wire plug that would normally go to the HEI distributor is not directly connected to the distributor but to the MSD 6AL ignition box with only one of the wires pinned out. I have not been able to locate an HEI distributor with provisions for a mechanical tach drive. The single wire coming from the male cap side of the distributor plug goes to the Tach terminal on the side of the MSD 6AL. Separate two wire plug from MSD 6AL to the MSD distributor. Not sure if the wiring is correct for this application without having the proper schematics.
Only other option is to find an electronic tach from later model and then use HEI distributor. Modifications were done prior to my buying the car and I don't have the correct documentation to troubleshoot wiring connections.
Car was running fine before problem started. We were driving down the highway on a warm afternoon and noticed a minor loss in power. Came up to traffic light and noticed the rough idle, similar to fouled spark plug or flooding condition. Drove at highway speed and every thing appeared okay, no odd noises, just minor loss in performance in lower RPM range.

[69MyWay]

Basics:

Believe it or not...a bad plug or plug wire could be your problem.

Also, if you don't already have a 3 wire O2...you need to get one. It will substantially help with running rich on cold start up, and get the computer into closed loop faster.

Go to www.tpis.com to learn more about the 3 wire (heated O2).

My money though, is on a basic simple tune up problem due to a defective or damaged plug or plug wire.

I mean...if it ran fine before with the cable drive tach, and now it is giving you trouble........it has to be something "tune up" related.

[patsnitrovette]

i agree with 69myway now that you say it run fine before ,also the heated 02 helped me out alot,plus i recently wrapped the headers down by the collector around the 02 sensor like a few other s have done and i get a better idle in gear at a light now,keeping the o2 sensor hotter.Do you have anyway of using a scanner on the car and seeing what everything is doing.Also you say vacuum is 15 psi,but is it flucuating12-15 or is it steady at idle.

[72Slydog]

I will recheck plug wires and plugs. Vacuum is steady at idle, (15 psi). Increasing rpms while watching vacuum gage shows steady decline then back to normal (15 psi). Throttle response is good. Did notice another clue during idle, rpms increase and decrease from 700-800-700....possibly a bad IAC??

If you haven't yet done a compression check, you're just spinning your wheels.

[72Slydog]

Compression check and fuel pressure check are next on the list. Three wire O2 sensor will be next purchase. GM dealer mechanic (corvette specialist) is betting that the MSD distributor is bad and should be replaced with HEI unit. Guess I need to start looking for a 75 or later electronic tachometer.

Thanks for all of your responses. Will keep you posted on the final resolution.

[72Slydog]

Patsnitrovette

What type of ignition are you using on your 73? I am using MSD 6AL ignition, MSD Blaster 2 coil, MSD 8.5mm wires, MSD 8571 distributor with tach drive. When scanning for errors, keep receiving code 42. According to MSD, I have to live with this problem unless I add GM HEI module. Are you having the same problem with your set-up and how did you remedy it?

Checked compression....190psi on all cylinders. Spark good on all cylinders. #6 spark plug did not appear to have fired. Replaced plug but did not seem to make any difference. Possibly a bad MSD rev limiter in ignition box is causing the performance problem.

[patsnitrovette]

Im using the dist. from the 87 trans am,small cap hei with remote coil(msd coil is on order),msd 6a box,so my harness is plugged into the dist. so the computer can control the spark.Works like it should.

[72Slydog]

Does the HEI distributor have provisions for the mechanical tach drive or have you converted your tach to electronic model?

[patsnitrovette]

it doesnt have a provision,but you can get the big style cap hei distributors with the coil in the cap with tach drive now,,performance distributors makes it,im converting my speedo and tach to electronic autometers.

[72Slydog]

Problem solved on performance....two badly clogged fuel injectors. Replaced with Accel 24#. Car runs great!

[patsnitrovette]

good to hear ,mine went bad also and replaced with ford motorsports 24lb