305tpi sd. its a 90 19lb injectors bought new(been this way for a year or so) I changed: plugs. wires, cap, rotor, coil, all intake gaskets..its an egr delete setup..i've properly set idle/tps and timing. problem it surges idle once its warm about 250rpms.. its driving me nuts! I have the timing set to 10degrees base. I've tried varying the timing to see if thats that problem. using my tech2 it dont' register any knocks.. the car runs awesome, great on gas, there are no drivablility issues at all. just the idle is driving me insane. does this sound like a sticking IAC? they are a bit expensive just to swap out. and I've read the possiblily of injectors? I would think there would be lean or drivability issue more pronounced. help me out here people i'm scratching my head

 

You've got a vacuum leak.

 

I have a 91 RS that I just bought and its doing just about the same thing. Starts right up, drives but during idle it surges up to about 1000 rpms then drops to almost 250 rpms and its like the engine catches itself and kicks it back up and just keeps doing it. it has a 350 block with the original 305 heads and tbi set up on it that the po did. Now the funny part is that I drove it all the way home no problem and even for the first few weeks then the other day I changed the plugs and not its doing that and I'm ready to scream.....its almost like I disturbed something but I have NO idea what.

 

Maybe your fuel pressure regulator diaphragm is leaking. Check the vacuum hose coming from the regulator for fuel. If you smell fuel at the TB (through the plenum), your injectors or regulator diaphragm may be leaking.

Hope this helps.

 

I just changed my fuel filter, took off my throttle body and cleaned the heck out of it and replaced the IAC tonight and that changed nothing. I'm still learning so I have to ask.....where is the fuel pressure regulater diaphragm at? lol. srry.

 

I have been down this road. Make a list:

Start looking for vacuum leaks. This is the first one people call, and it is not always the answer.

If you have no leaks, test the injectors.

If the injectors check out, go through your sensors. Start with the throttle position sensor and work your way down the line.

If none of that does it, you may want a shop run monitor it on a computer.

 

Check for a vacuum leak by spraying starting fluid or carb cleaner around the intake, plenum, and runners, when the car is running. The rpms will increase noticeably if you spray fluid over a vacuum leak. Check the vacuum hoses for leaks by grabbing them with your bare hand; if they leave your hands black, replace them.

(Assuming you have a TPI) The fuel pressure regulator diaphragm is inside the fuel pressure regulator. It is a pain in the butt to check because you have to pull the plenum and loosen one of your runners. It is attached to the fuel rail under the intake plenum, on the passenger side, next to the fuel pressure test port (looks like an over-sized tire schrader valve screwed into the top of the fuel rail).

I solved my surging idle with new vacuum lines, new injectors, new regulator diaphragm, removed CSI, reset IAC, and return timing to factory specs. But I never would have fixed anything without a scanner, it is a valuable tool.

Hope this helps.

 

My RS is TBI but I did change my fuel pressure regulator diaphragm tonight and cleaned my injectors and nothing changed still surging. I checked the MAP sensor when I was at the parts store with a new one and that didn't change anything even though it was REALLY weird because when you disconnect the vaccum from the plenum on the TB to the regulator OR just unplug the harness from the MAP sensor the car levels out and just slowly dies..........thats why I tried changing the pressure regulator. Like I said....same thing though. I'm going nuts!!!!!

 

Thats too bad. If you have eliminated vacuum leaks, I think you need to hook up a scanner to the ALDL port and look for problems with your sensors. As mentioned above, check TPS, CTS, and O2. Do you know anyone with a scanner? You can get or build an ALDL cable to hook your computer up and use it as a scanner.

You could also check the sensors with an digital voltmeter/ohmeter. Check to make sure the TPS moves smoothly when you move the throttle. Make sure the TPS is set to the proper minimum/idle voltage. Measure the CTS ohms when cold and again when hot, just to make sure they are different. Measure the voltage on the O2 sensor wire, it should vary between 0.1 and 1.0, it should not just stay at one value when the car is running.

Hope this helps.

 

I think that you are right about it being a good idea to hook up the ALDL to the OBD1 and laptop so I can see what the code is that it is throwing. I don't know anyone that has one but I could buy one with the program off of the net. I did find one thing that was peculiar today with the engine running unplugging the CTS made a substantial difference in the idle and the surging. I didn't install it yet but, I bought a new CTS because it isn't a expensive part. I now know where the surging is coming from.......The ECU is telling the injectors to spray WAY to much gas in spurts choking the engine down to like 250 rpms then when all that gas is burned kicking the rpms up to like 1000-1100 rpms its sprays another huge burst of gas. This cycle just continues over and over hence the up and down surging......it sprays the gas SO hard in those burst that you can hear it spray off of the throttle plates......its nuts. Has to be a bad sensor sending faulty signals to the ecu confusing it somehow.

 

ok......I'm going nuts. I took the car to a shop.....they told me that it is throwing a code for the knock sensor and the Electronic Spark Control. They also said that there was a vacum leak at my intake manifold gaskets. I changed them last night and it is still doing it. IDK what the hell to do. This is getting crazy.

 

Do you have a single wire O2 sensor? I chose to upgrade to a heated 4 wire sensor because it has a dedicated ground. It is possible that your O2 isn't heating up enough to function properly.

I still think a scanner will pin-point exactly where the problem is. I built an ALDL cable in an afternoon and two trips to Radio Shack. If you know how to use a soldering iron, you too can build an ALDL cable. Check out this site for a free download and instructions on building a cable...

http://winaldl.joby.se/

Hope this helps.

 

Set your timing to 6. I had the same problem and went through the same process as you. When I finally was able to check the timing I was only slightly advanced from factory recommendation, I believe around 10 degrees advanced. When I dialed it down to 6 degrees advanced, my surging disappeared.