I have a 92 Camaro Z28 TPI 305/5-spd.

My problem is I have a hunting/surging/rough idle the idle varies about 500 rpm and doesn't really smooth out as you accelerate either, and occasionaly stalls when i come to a stoplight.

Recently I have replaced the plugs, wire, cap, and rotor. All my vacuum lines appear to be fine no cracks or leaking, atlhough I do have a small leak at the throttle shaft. A smoker was used in the vacuum system to look for leaks and some smoke poured from around the throttle shaft/linkage.

I believe it is a speed density system... It has no MAF sensor ... just a MAP sensor. I am currently enrolled in some automotive courses at a community college so I was able to test some things, the TPS and MAP and CTS all appear to be working properly but the IAC counts were high i believe... cant remember for sure but i think the counts were 82 at idle (these were all checked with OBD1 scan tool).

One of my teachers gave it a look and said that there was a problem on the right bank of cylinders... he pulled the spark plug wires one at a time and he said that there was no RPM drop on the right bank of cylinders, the left bank responded to no spark and RPM dropped. And since the ECM fires a bank at a time he thought it could be the ECM. So we checked it then the ECM turned out to be fine.

So next we tried checking the resistance of the fuel injectors and only 3 of the 8 injectors were within specification (15.8-16.6 ohms) and guess what all 3 of the good ones were on the left cylinder bank .... one on the right bank had a res. as low as 4.9.
So that would make sense that the left bank of cylinders responed to pulling the plug wire?

Then we checked the current draw of the injectors on a scope... My teacher explained to me that I should see a smooth ramp of an increase in amps as the injectors open... then an abrupt drop as it closes. A few of the injectors kinda looked like that but none were really as they were supposed to be. Most of them had a straight line upward then straight across then straight down.
So they were opening too quickly?

Next I removed the throttle body and cleaned the IAC passages and throttle plates and so forth. Disassembled and cleaned the IAC itself and reinstalled everything. This didn't seem to make any difference.

What my question is can it be my fuel injectors and this small vacuum leak at the throttle shaft for my problem? Or is it something else or a combination of things... Is there anything else I can check to be sure of this, before I cough up a lot of cash on injectors.

Thanks for any help guys. This forum is great

 

Bob,

Lots of description, information, and test data. That's good.

The injectors are a purely digital device. The current waveforms are going to be a relative square wave as you describe, unless you have the sweep/raster speed of the scope cranked way down to about .01 mS. Most strictly automotive oscilloscopes don't even have that capability, nor trigger level adjustments, sweep sensitivity, holdoff, etc. All you're going to see is a positive ramp that is almost straight up, a holding current (the straight line across the top), and the turn off. By your description, the current waveforms seem normal.

What is more of a concern, however, is the pull-in and holding current level, and not the waveform. If you scaled the display, you should have measured a lot higher pull-in and holding current on the lower resistance solenoids.

Your '92 most likely has Multec injectors. They are notorious for solenoid insulation failure from exposure to solvents. The factory specification for acceptable coil resistance is from 16.2-12.7 ohms. Once you get below 12.7, the inrush current can be more than the ECM driver transistors can tolerate. If the current gets too high, the ECM will shut down the drivers to prevent destroying them. Sometimes this works, and sometimes the transistors fry anyway.

So my first point nwould be that you have a low-resistance injector solenoid that is below the safe service limit, and it should be replaced.

From an operational point of view, you also are experiencing rough idle. Chances are that one of more of the injectors has an issue with correct flow, from either clogging inlet screens, sluggish solenoid cores or pintles, damaged spray tips, and/or failing check valves. Despite what some erroneously believe, resistance tests of the solenoid coils will reveal absolutely nothing about the flow characteristics of the injector. The only method to determine this is to actuate the injector under controlled conditions and measure the output flow.

The power balance test performed by your instructor is a good relative indication of injector flow, and may help isolate the poorer-performing injector(s), but replacing or servicing one or two injectors is only delaying the inevitable. Coupled with the fact that you have a failing solenoid coil on at least one of them, I'd be planning to remove the injectors for service (cleaning and flow-matching) and replacement of the lower resitance units.

Beyond that, with the known issues attributed to these earlier Multec injectors, you may be better advised to shop for a used set of Rochester/Bosch injectors, have them serviced and ready, and take a good long day to exchange them. New injectors are pricey, and used sets can often be had for less than $100.

 

Vader,

Thanks a lot for your help.Vader, I guess I will be replacing fuel injectors.. I have looked around and on www.lindertech.com I can get some refurbished injectors (Bosch) for around $200/set... I have seen all the steps they perform on the injectors and it seems very thorough, My other alternative would be to get some Accel injectors for about $230/set but I think factory style injectors would probably work better.... So hopefully a complete set of injectors will cure the problem I'll let you know how it goes. Thanks again Vader.

-Bob