Driveability problems! Help!

[Silent Z]

This is my first post so here it goes. My 1994 Z28 has 82,000 miles on it and I have had it for 1 month. Once in a while the car will hesitate at 55 mph @ temp.200, 1500 RPMs. If I floor the car it goes away as if its at a bad spot on the throtle position sensor. But then it will do this at any speed. If I put it in nuetral and rev the engine it feels fine. The next day it wil drive perfectly. I am by no means an electrical guru, but I hooked up the shop Tech II. The SES light is not on but it has a code 0018 for injector unsual voltage. I am an impulsive person so I just bought a map sensor and a tps for the car and will install them in the morning. I hear so much about Opti Spark, but I dont know where to start and I have never learned how to check injectors. Any help would be great!

[Ian]

WELCOME!

I am hoping for two things right off the bat. 1) I hope you didnt buy a map sensor because a 94 wouldnt have one OR 2) you have a 93 and you really do have a map sensor.

It wouldnt be a dead spot on the TPS because that would do it all the time and not just at one speed. because you have a code for unusual injector voltage, just check the wiring around the injectors for any signs of damage. burned or melted wire insulation could cause it to ground out every now and again. check that and check simple stuff like plug wires and plugs.

Mike would probably know better than I would though, try asking him (forum name: 9secfirebird)

[Silent Z]

Thanks I'll look into that in the morning. The car has a map sensor and its a 1994. maybe the motor is a left over 93.

[Pampered-Z]

I would first pull the ECM fuse for a few minutes to clear the codes then reinstall and drive the car to you can see if the code returns. make sure your not chasing an old problem because of an old code?

As Ian stated, check the wiring. Check that all the clips are secure, and there have been a few cases over the years where the wiring back near the ECM and the mount for the hood shock/lift and heater box has rubbed thru and caused intermittent missing. With the car running move the wires around and see if the engine stumbles.

What condition is the car in? Do you know if it was recently tuned up? With 82K I would first do a tune-up: Fuel filter, plugs and wire and probably 02 sensors. When the 02s start to fail they can give the ECM all types of odd problems and not set a SES code.

Sticking EGR valve will also car the car to run poorly. One simple test is to put the car in gear and power brake it. If the EGR is sticking open slightly the car will shudder badly under the load.

I don't think your having an Opti issue. most times when they fail it's more of a constent problem, missing at high RPMs is often the first sign, also backfiring or very bad stumbling, normally not occasional stumbling or missing.

John

[Silent Z]

The car is in great shape. I checked the injector harness and all was fine no melt and all were secured. I looked at the PCM and your right the harness does rub on the heater box and A/C line. I didnt have the time to pull it out to inspect the wires though, but what I could see didn't look that bad. I was looking at the cap, rotor, wires, and plugs... what a pain in the ass. Do you usually remove the Alternator and power steering pump to do the plugs on the right side? And do you remove the water pump assembly to do the cap and rotor. This is nothing like my old 3rd Gen.

[Silent Z]

The car has Bosche Platnim 4 tips plugs and autolite 7.5mm wires. I hear those plugs are crappy what do you guys think?

[camaro2you]

I heard the same thing about them. i like ac delco plugs and taylor wires.

[skorpion317]

Originally Posted by Silent Z

The car has Bosche Platnim 4 tips plugs and autolite 7.5mm wires. I hear those plugs are crappy what do you guys think?

get rid of them. the extra electrodes shroud the spark, causing incomplete combustion. look in your owner's manual for the recommended plug.

[Silent Z]

I just bought Delco platinum OEM replacements plugs. I just haven't had the time to instal them yet! Thanks for the info.

[Fastbird]

Originally Posted by Ian

I am hoping for two things right off the bat. 1) I hope you didnt buy a map sensor because a 94 wouldnt have one OR 2) you have a 93 and you really do have a map sensor.

All LT1's have MAP sensors. The 93's just utilize them moreso than the rest.

You said that you hooked it up to a Tech II. I'd suggest driving the car around (or have someone drive it with you watching the tech II) and keep an eye on some specific things:

Injector pulses
O2 mV
And keep an eye on the tach signal to see if it goes crazy right about the time the car has it's issue. I've seen it before where nothing shows a problem but there's a crazy surge or stumble, and while datalogging the RPM line goes nuts right before the issue, indicating an ignition issue.

Just look for anything out of the ordinary. Normally that type of stumbling that you described is ignition related, but I wouldn't be so quick to think opti because usually at that low of an RPM if you put the car under a heavy load (mash the gas) if the opti was on it's way out you'd REALLY experience some stumbling and hesitating.

How old is the coil??

[Silent Z]

The coil is the original 83,000 miles on it now! What is a good fuel pressure reading, my electronic service manuel doesn't give one. My readings were like 37 to 40 psi!

[Silent Z]

I just changed the driver side spark plugs. They were black covered with carbon. So I guess skorpion317 was right about them. Now how would you guys do the passenger side?.... Pull the starter, alternator or what? I did find a bad vac. line from the EVAP canister purge solenoid valve going into the throttle body. I just ordered it from the dealership $7 for a 3 in. hose hands up I just got robbed. LOL!

[Pampered-Z]

Fuel pressure seems fine.

Yes, for the pass side it will be easier if you remove the alt and starter and belt tensioner. The biggies PITA is getting the clips that hold the wires in place open! There is two that hold all 4 wires: One near the opti by the PS pump pulley, and the biggest PITA is the one on the side of the alternator bracket. There are two or three other clips that only hold the wires for the 6+8 cylinders, but they are earier to get to.

[Silent Z]

Thanks for the info pampered z. I'm going to finish this up friday.

[Silent Z]

Well I did the plugs and wires last week and no luck! I have fixed a few vaccum leaks. The kid I bought the car from payed someone to do the CAI and airfoil. When they reinstalled the elbow onto the TB they jamed it on making a sizable intake leak after the MAF sensor. The gap was on the underside so you couldn't see it. I thought that would fix it, but again I was wrong! I was watching the TechII and I noticed that when I hit the gas in park the timing would retard. I did the same thing on my brothers 1994 LT1 Trans AM and his didn't do this. So I just installed a LT4 knock module and no more knock timing retard! I still have to drive it home so I hope this fixes it. :P

[Silent Z]

Well again I was wrong this is happening way to often. The timing still retards when at operating temp. I used the snap-on vantage scan tool and my IAC and MAF are fine, but I did these tests while engine was cold. Should I do these tests while it's hot? I know it was a waste but I have replaced the Throttle position sensor, MAP sensor, Knock Module, and Air Intake Temp. sensor I am at a loss. I know when I it fixed I will have no worries becasue its all new. Is there a way to test the PCM? I would hate to replace the PCM or Opti Spark and find out they were fine because those items are not cheap. All of the stuff I replaced sensor wise only cost about $100.00 so I have not spent a fortune. I have a new MSD coil on the way! Any ideas?