Higher than normal Idle, and occasional stumble

[Billy Mild]

I have been struggling with a higher than normal idle since I pulled the plenum a few months ago. When I pulled the plenum the following things were replaced: Plugs, Wires, Fuel Inejctors, and fixed any remaining vacuum leaks.

Sometimes my idle will hang around 1500-1700 RPM. If I stab the throttle really fast it will go back down to 900 RPM. I have noticed that when the engine is still cold, not fully warmed up, the car will sometimes stumble around the 2500 RPM when I go more than 50% throttle. If I go WOT it pulls through it without issue. There seems to be a sticking point around 50% throttle. Once the car comes up to full temp this issue becomes less noticeable. I haven't tried to spray the throttle blades with lithium grease or anything.

I have read a few old threads concerning the idle issues. It seems that may be related to my Throttle Body needing a service, or there could be a vacuum leak somewhere. I don't think there is a vacuum leak since my secondary vacuum pump will run for a few seconds with the key on, and shuts off for a long time. I tried to do the IAC reset procedure. I drove just a little bit after that and the idle did seem better, but still not perfect.

Any ideas?

[Paul Workman]

I'm betting it is TB/linkage cable related, in that if you poke the throttle the idle will sometimes come down to normal - which it would not if it were a plenum gasket leak or the like.

First, make sure the connections in the TB connector are clean and the connector itself is attached properly. Then, with the switch on, but the engine not running, the TB voltage signal at ilde position should adjusted to .54v, ± .08V (.46 to .62 volts)*

*1990 FSM; Driveability and Emissions, sec. 6E3-C1-19 (TPS) Adjust

Note: The voltages may might vary for other years, I don't know.

Next we want to verify the IAC valve connection is properly seated. And, if it is, then pull the IAC valve and inspect the pintle and the air horn seat to be sure they are not impeded with carbon deposits, or the like and the pintle moves freely before reinstalling it.
If all checks out to this point, then the linkage and connections, and the throttle plates too should be checked out to verify proper motion - NOT sticking, and that includes the idle throttle plate as well.

Note: There is a waxy seal spread around the throttle plates to help seal them when closed. It must NOT be removed, or idle issues will prevail. Be careful to not disturb it.

While idling, place a book or the like across the open air horn. If it continues to idle, then for sure you have a leak - likely a plenum gasket got dislodged or the like. But, that is more for reference at this time, until you've eliminated the TB/IAC functions first.

BTW, the vacuum pump not running/shutting off does NOT mean there is no leak elsewhere, e.g., say the plenum gasket, for one. The secondary actuator circuit is isolated from the plenum by check valves. The pump will "charge" the vacuum circuit initially and then shut off, even with ZERO vacuum in the plenum, e.g., such as when the key is turned on but the engine is not running, OR at sustained WOT operation.

[XfireZ51]

Paul,

The TPS v should be .53-57v with key ON/no engine start.

[Paul Workman]

Originally Posted by XfireZ51

Paul,

The TPS v should be .53-57v with key ON/no engine start.

Not to split the atom, but I wonder if the voltage recommendation varies w/ model year? But, in any case, mid .5s volts appears to be close and easy to remember, I recon.

[XfireZ51]

Originally Posted by Paul Workman

Not to split the atom, but I wonder if the voltage recommendation varies w/ model year? But, in any case, mid .5s volts appears to be close and easy to remember, I recon.

Not sure why it would vary by model year. Same TPS, same values for idle in the calibrations. There is provision for hysteresis in the cal also.

[Billy Mild]

I don't have a scanner. Is there another way to check the the tps voltage and then IAC counts?

[mike100]

Originally Posted by Billy Mild

I don't have a scanner. Is there another way to check the the tps voltage and then IAC counts?

IAC counts can't be determined without a scanner, but the TPS voltage can be manually read by carfully probing the center wire of the tps connector with a sharp instrument or paperclip possibly and then to ground on the other side of the meter (or to the negative wire on the edge of the 3 wire connector). Back-probe the connector with the ign off and then once the meter is all hooked up, then turn it on.

[Billy Mild]

Well it looks like I am back to square one. I drove the ZR1 quite a bit yesterday and my secondary pump would only prime the system then never run again. This morning (34 degrees) it would prime, then stop running for 5 seconds, then start up again. I guess I have a vacuum leak again under the plenum.

Is there anyway to fix these vacuum leaks for good? I'm considering using silicone tubing like I used on my old 944 Turbo. After I used that stuff I never had vacuum leaks again.

[Dynomite]

Originally Posted by Billy Mild

Well it looks like I am back to square one. I drove the ZR1 quite a bit yesterday and my secondary pump would only prime the system then never run again. This morning (34 degrees) it would prime, then stop running for 5 seconds, then start up again. I guess I have a vacuum leak again under the plenum.

Is there anyway to fix these vacuum leaks for good? I'm considering using silicone tubing like I used on my old 944 Turbo. After I used that stuff I never had vacuum leaks again.

I am assuming you are removing the Plenum........What usually happens is that the little rubber connectors get hard or loose which you can use your silicon Seal. Some use a nylon strap tightened around the fitting. Others like myself.....find a nice rubber connector that fits tight. Jerry has some of those connectors in stock. Vacuum System Components

Finding A Vacuum Leak

Now if you really want to have some fun and be able to remove the Plenum in less than 10 minutes and fix those secondary vacuum leaks in just a few minutes....... ELIMINATE TB COOLANT

See Item #4 LT5 Eliminated Systems

[Billy Mild]

Originally Posted by Dynomite

I am assuming you are removing the Plenum........

Finding A Vacuum Leak

This will be my 6 or 7th time taking the plenum off. Granted 3 of those times were within the same day since I forgot something or left a connector off.

I should be able to get this done pretty quickly. I'm assuming somehow one of the hoses vibrated off somehow. I may end up replacing the rubber ends with silicone hose. Follow this procedure to make sure they can't come off.
http://www.turbo944.com/bbs/config.c...ames;read=8056

Might be crazy, but it works. I am tired of the vacuum leaks.