Introduction
I was having some "Rough idle" problems on my 89 GTUs and I thought I would share my experiences of how I resolved them. I'm writing this as a FAQ of what to do when ALL ELSE FAILS! You may want to check some other "simpler" things (like the TPS) before you follow the steps below. Feel free to add any other advice you may have on this topic. Note: This FAQ is written assuming you are using the stock ECU and all emissions equipment is intact.
Vacuum leaks, vacuum leaks, vacuum leaks
You will never be able to set a rock solid idle if there are any vacuum leaks. Cracked vacuum lines, old gaskets, bad o-rings/grommets around the injectors are all suspect. The only way to fix all the vacuum leaks and eliminate all doubt is to tear everything down to the bare block. Clean off all old gasket materials and replace with new gaskets and new vac lines. It's also a GOOD IDEA to apply a light coating of silicone to both sides of every gasket.
TIP: My favorite way to remove old gasket materials is 3M Gasket Removal Pads.
Potential vacuum leaks, often overlooked:- The large rubber hoses from the BAC and AWS valve. These hoses become loose over time and often need hose clamps to make a good seal after they've been removed a couple times. Especially where they attach to the intake snorkel.
- The throttle body spacer. Remove it and put a small amount of silicone gasket maker on both sides. Be careful not to block any of the vacuum passages on the spacer.
- the ACV gasket and check valve. This gasket doesn't come in any of the engine rebuild kits and can often be hard to find at the dealer. Don't overlook this gasket as a vac leak source. Especially when using an ACV block off plate.
- The brake booster vacuum lines. Especially the one directly above the exhaust manifold/turbo.
Resistance Values – Operational Checks
While everything is apart, spend the 15 minutes to verify operation of all of the sensors and solenoids. For the operational checks, apply 12V and listen for "clicking"
- BAC: Resistance: 10.7-12.3 kOhm. Operation: Apply 12V to check
- AWS Solenoid Resistance: 9.3-11.3 kOhm. Operation: Apply 12V to check
- Air Intake Temp Sensor (In Manifold): @ 77 Degress, 33 +/- 4KOhms.
- Water Thermo Sensor Resistance: @ 68 Degrees, 2.5 +/- .2KOhms
- All Vac Solenoids (Operational Check): Apply 12V to check, listen for click. It's always wise to blow through the solenoid during this test to check for leakage.
- ACV (Operational Check x2 plugs): Apply 12V to check, listen for clicks.
- TPS: Attach an Ohm meter and check for flat spots (see [TPS Adjustment]).
Injectors
If your injectors have never been cleaned, do it now. My favorite place to send them is Marren Fuel Injection.
[»http://www.injector.com/»] Cost for 4 injectors is around $100 bucks and worth every penny. They will replace the caps and o-rings, however you will need to purchase the upper and lower injector grommets. *TIP1* Apply a thin coat of oil to the injector grommets/o-rings before installing them so they don't tear.
TIP: Check the pulsation dampener for leaks (loose screw) and the fuel lines for cracks.
Throttle Body
Clean the throttle body and get all of the carbon build up off the primary and secondary plates. Verify the clearances of the primary and secondary throttle plates. (See Workshop manual – Fuel and Emissions Control) This step is VERY IMPORTANT! Get the clearance of the primary throttle plate perfect. The clearance between the throttle plate and the bore should be between .02" and .03" inches. (for all 5-speeds)
TIP: If you don't have proper feeler gauges to take this measurement, use dial calipers to find a piece of wire that is the proper thickness.
Reassemble
Put everything back together. Pay special attention to the connections between the lower intake and the intermediate intake. Tighten all bolts evenly. At this point, you have eliminated all sources of vacuum leaks, verified all electronic controls, and adjusted the throttle body. You are now ready to start the car and let it warm up.
TPS
Once the car is fully warmed up, turn it off and check the TPS. PERSONALLY I prefer the LED Test lamp method to check the TPS. For this method, see
[TPS Adjustment].
Conclusion
At this point, if you've done EVERYTHING correctly, your idle SHOULD be rock solid. If the idle still pulses, (IE: Fluctuates between 750-1500 RPM) you'll need to recheck for vacuum leaks and ensure the throttle body plates are set right. Also check your engine grounds and fuel pump pressure. Check the alternator for proper voltage (Above 14V with the engine running) and finally check your ignition system.
See Also...
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