CamaroDon

I’ve been gleaning a lot of useful information on this message board about idling issues, but I haven’t yet hit upon the culprit for my particular problem. For the past few weeks, my 1989 Camaro RS (2.8L V6, 5 spd manual) will experience an abrupt drop in RPM (down to around 500) after a couple of minutes at idle (usually at a stop light for instance), running rather rough for about 20-30 seconds, after which it returns to normal idle with smooth operation. During those 20-30 seconds, the Service Engine light will come on, then go off when the engine returns to a normal idle. When it first began doing this a number of weeks ago, it would set a variety of ALDL codes, including “13” (O2 sensor circuit), “24” (vehicle speed sensor), and “44” (lean exhaust). The Haynes manual advises to disregard any “24” code which is set while the vehicle is stationary, so I haven’t messed with the VSS. I’ve replaced the O2 sensor, MAF sensor, and for good measure I replaced the Idle Air Control Valve, which seemed like a potential suspect after perusing the message boards here. With no improvement to the above symptoms, I took it to my mechanic this week, who looked it over and couldn’t find anything obvious, nor was he coming up with anything on his diagnostic equipment. He said it didn’t act out for him at all, which I find odd, since it does it for me every time I let it idle. Anyway, he suggested that it may likely be something in the fuel injection system. He cleaned out the throttle body just to see if that might help. It has actually been running a little worse since he did that, and I’m now experiencing a wider (1000-RPM) fluctuation at idle now, still accompanied by the Service Engine light whenever it drops below around 800-900 RPM. Yesterday I let it idle in the driveway for four test periods, disconnecting the battery between tests to clear the codes. It performed as described above each time, with the idle speed fluctuating between 800-1500 RPM, then kicking in to a low, rough idle for 30 seconds or so, causing the Service Engine light to come on. The first time it set just a “44” code; the second and third times, it set “24” and “44” codes, and the fourth time it set no code at all, even though the Service Engine light came on all four times. Over the entire several weeks of this problem happening, it seems the “44” (lean exhaust) code has been the most prevalent.
I replaced the injectors and fuel pump about 60,000 miles ago. Going on my mechanic’s recent suggestion, I guess my next step now is replacing the injectors and testing the fuel pressure. The car gets good mileage and seems to have full power (no stumbling on acceleration, and runs smoothly at drive speeds), so I’m not overly confident that the fuel delivery system is the problem, but I’m running out of things to check. I snooped around for vacuum leaks but haven’t found anything obvious. It doesn’t seem to matter whether the engine is hot or cold; once warmed up the engine temp stays within normal operating parameters. Am I missing anything? Thanks to all who have any thoughts.
Don

m16a2r

did you check your throttle position sensor? I replaced mine and it helped with an idle issue on my car

CamaroDon

I haven't checked the TPS but that's a good idea. Thanks!

CamaroDon

Just an update to my original post:

I pursued the one suggestion posted, by m16a2r, and found that my TPS setting was off by .02 volts from the manual specs, which I corrected. The idle and intermittent SES light problem persisted, still setting a "24" code (VSS) which I have ignored per the Haynes manual since the car had been stationary. (MAF, O2 sensor and IAC are all new.) Today I decided to check the timing, and found that not only was the timing off (18 BTDC; sticker says 10), but the distributor was loose enough to turn by hand without much effort. After correcting the timing, much of the fluctuating idle problem has gone away, and the SES light stays off now. Idle still fluctuates between 600-1000 RPM, without much difference between a cold and hot engine, but isn't quite as obnoxious about it. Tomorrow I'm going to replace the distributor cap & rotor, plugs & wires, fuel filter (and CTS for the heck of it), and check fuel pressure while I'm at it, and see where that gets me. I'm holding off on injectors until I'm more certain I need to throw money at a new set. And of course, I'm still soliciting other suggestions!

CamaroDon

Well, after a week of searching for the cause of my fluctuating/rough idle problem in my '89 RS 2.8L, I stumbled onto the problem. The EGR tube was connected to the intake plenum by only one bolt, and the gasket had broken away in one spot, creating a vacuum leak. I only discovered it after trying the carburetor cleaner trick on my vacuum lines, which occured to me to try after spraying cleaner on the throttle cable connector on the other side of the throttle body. As I was cleaning it, enough spray had made its way over to the EGR tube connection, as my idle smoothed out significantly. The missing bolt wasn't very obvious thanks to all of the components above it, but I should have checked more closely for vacuum leaks to begin with. After installing a new gasket and bolt, my problem went away.

Just thought I would pass that along in the event it helps someone else with a similar problem, and saves them some time and grief.