Like the title says my 93 Z28 has developed a very annoying stumbling when at cruising speeds. The car starts up fine, no problems at all, but when it reaches cruising speeds and I try to hold it there it bucks and stumbles like crazy. At wot everything is fine tho it takes off normal. In the past 5 weeks the car has had a new opti, wires, plugs, coil pack, ecu, cleaned the injectors, new wire harnesses, vacuum testing...literally everything besides replacing the engine. The only thing weird is the gas tank's vacuum pressure is low, 40psi instead of 44-45psi. Has anyone ever fixed this problem??

 

I'm going to assume you measured the fuel pressure at the fuel pressure test connection, behind the passenger side rail, and not at the "tank". Not sure what "the gas tanks's vacuum pressure" even means. Was the 40psi figure at idle? With or without the vacuum compensation line connected to the fuel pressure regulator?

Didn't notice the ICM in the list of parts you replaced.... or is that what you are calling the "coil pack" (coil + ICM)?

 

A faulty tps can cause bucking while under steady driving conditions.

 

I assume the pressure reading was taken from there that's what the shop is telling me anyways, it was at idle tho and connected to the line is what theyre telling me. The icm was replaced with the coil sorry for the loose term. Could the egr valve be faulty and causing it ive seen a few people on here have that problem.

 

40psi with the vacuum line connected it OK. 40psi with the vacuum line disconnected it just outside the GM spec of 41-47psi.

EGR only works at low RPM (less than 3,600) cruise conditions. If its letting air into the intake rather than exhaust, it could cause a stumble. May also be a voltage spike in the throttle position sensor (TPS). Could be a problem with the MAP sensor, since that is critical to the A/F ratio in your 93's speed-density control system.

 

Alright were gonna look at the EGR first since it is happening around 1800-2000 rpms, how would u diagnose a bad map sensor? 93 cars dont have a maf cause its speed density right? Is that going to make it harder to find a problem?

 

You need to put a scanner on it, and watch the MAP readings. Even better, get scanning software, and do a data log that includes the stumble.

TTS DataMaster makes a 93 version of their sfotware, free download, 20 free uses.

http://www.ttspowersystems.com/DataM...ownloads2.html

Cable:

http://www.akmcables.com/

Correct.... 93 does not have a mass air sensor. The ECM calculates the mass air flow rate using RPM, displacement, VE tables, MAP and IAT.

 

Is the bucking happening at speeds that one would normally spend a lot of time at? Like 35,45, 55..etc... the contacts of the variable resistor of the tps do wear out at the spot where they are mostly used.

 

Sorry to bring this post back from the dead but here's an update after 5 weeks and 3 different shops. Originally when the shop installed the new computer they used an aftermarket piece and not a GM one. According to a "GM expert" at the new shop this is why my car will not run correctly and is the solution to all my problems. The only catch is that this GM computer will take 4 weeks to get, does this sound legit to anyone?

 

They're telling you your car was running an aftermarket ECU? There are some out there - FAST, Accel DFI, MoTeC, Electromotive, etc., but I seriously doubt someone installed a high $$$ aftermarket system and didn't bother to tell you or charge you for it. I don't know what else they could possibly mean by an "aftermarket" ECU, and there is no reason they can't go to any junk yard and pick up a replacement, or get a rebuilt unit from an auto parts store in a day.

Sound..... odd.

 

Well it's a long story. The car originally went in to get a new radiator and the shop did not put on the radiator hose tight enough, it bursts while I'm driving it home and kills everything electrical under the hood. So for the past 12 weeks I have been waiting for them to fix the thing and now I get told that the ecu they originally put in to replace the dead one was an aftermarket and it's not working right. I have no idea how it takes 4 weeks to get a stock ecu for an 93 lt1. I just hope I get it back I hate driving my civic.

 

Not to add insult to injury, but I can pretty much guarantee you there is absolutely NOTHING wrong with your original PCM. Had you been able to do some Datalogging like Fred mentioned in post #7 it would have been invaluable in diagnosing your problem.

 

And they are telling you that a leak in the radiator hose soaked the PCM?

Sounds....... odd.

Personally, I wouldn't believe anything they are saying.

 

My guess is they wanna wait 4 weeks so that they can cover the damages they caused...