88 Supra N/A refuses to idle

[btwilson86]

I have been fighting a problem on my 88 7MGE Supra, and I am stumped and looking for suggestions on things to try now.

This car refuses to idle with the throttle plate completely closed. If I crack the throttle just a hair, it will continue running on it's own, but starts hunting (revving up and down over a couple hundred RPM's, like it has a vacuum leak). Off idle up through to the red line, the engine seems to run well (albeit slightly underpowered versus other N/A's I've experienced). In attempting to fix this problem, as well as fixing other problems as I find them, I have changed several parts with other used parts that have sat for over a year (but came off a good running Supra).

*I feel it may be important to note that the problem I have now came on suddenly, as it was running fine when I got on the freeway one night, and when I came to the end of the freeway exit it no longer would idle.*

These are the parts that have been replaced since this problem has started, as well as some checks/inspections I have done:

• Checked for Engine Codes
• Deleted No. 3 Cylinder Head Cover with drain plug gaskets- old one leaked bad (originally thought this was the cause of my problem)
• New BCPR5EP11 NGK Spark Plugs (Laser Series Platinum #2097) - replaced twice
• New ACCEL Spark Plug Wires
• New Borg Warner Distributor Cap
• New Borg Warner Distributor Rotor
• New Intake Manifold Gaskets
• New Exhaust Manifold Gaskets
• Used Intake Manifold
• Used Throttle Body
• Cleaned internals of intake manifold and throttle body
• Used TPS (properly calibrated per TSRM)
• Used ISCV
• Used ECU
• Used Engine Wiring Harness
• New O2 Sensor
• Fixed Exhaust leaks on Pacesetter Header
• New Exhaust Pipes and Muffler (Muffler is where Cat should have been, car didn't have one when I got it. Piped the exhaust out the side for now)
• Verified Cam timing is correct (Both cams were about 3/4 of a tooth off, best I can get them now is about 1/4 tooth off- may need to replace timing belt)
• Verified vacuum lines are routed correctly and are in good shape (I've actually done this about 10 times thinking I missed something as I still feel like this could be a vacuum leak issue)
• Removed and installed Distributor
• Set Ignition Timing to 10? BTDC (this one may not be right- since the car doesn't have a stable idle, I don't know if the ECU attempts any timing adjustments to try to stabilize the engine speed)

Some notes and observations I've made on how the car is running:
On a cold start, the engine fires right up, idles at about 1400 RPM, slowly drops idle over a period of about 30 seconds, starts to get rough and seem to misfire at about 1000 RPM, sputters as the RPM's continue to drop, and stalls out. Restarting the car will yield the same results, only it all happens a bit quicker the warmer the engine gets.

On a warm start, the engine fires right up, hits about 1200 RPM, and immediately dies. Sometimes it will sputter and attempt to save itself once or twice before stalling.

If I unplug the electrical connector for the ISCV, then start the engine while it's warm, the engine will slowly rev up to about 2200 RPM where it then drops down to about 1200 RPM and continues revving up and dropping between these 2 points. If I plug the ISCV connector back in while the engine is running, the engine will idle around 1100 RPM with a bit of a lope to it (sounds like a domestic V8 with a cam upgrade). This does not store nor indicate any engine diagnostic codes.


At this point I am driving the car somewhat regularly, and am feathering the throttle to keep the engine running while stopped. I do not have any engine codes stored (the bulb does work, get the "all clear" flashes when I jumper the ports in the diagnostic box). I feel that my ignition system is in good shape (unless the Distributor's CPS signal to the igniter is bad at low RPM). My personal theory at this point is that I am either getting too much air (vacuum leak) or too much fuel/improper atomizing of fuel.

Anyone have suggestions on what to try next?

[btwilson86]

So... bump?

I know I've replaced a lot of the components that could affect idle, but if someone else has any suggestiins I'm all ears. Any TCCS gurus want to chime in? Lol

[Busted Knuckles]

Just throwing this out there, but how's the condition of the air intake between the KVAM and the throttle body? There's a vacuum line that comes off the air intake to feed the power steering pressure control valve, IIRC.

[btwilson86]

So far I've sprayed it down with carb cleaner trying to get a change in idle, and haven't noticed any difference. I have checked and made sure the vacuum line is connected on both ends there. Ill take the intake duct hoses off and visually inspect them for cracks or anything else wrong, I think I have access to another set if I need to replace them.

[btwilson86]

Well, I found the problem thanks to Busted Knuckles. After removing the intake pipes, I found that the header has made contact with the Helmholtz resonator and melted a hole in it. I've temporarily wrapped it in duct tape to seal the hole, and the car idles nice and smooth now.

Thanks for the help!

[bills1988]

Glad to see you got it fixed. Here is what caused my 88 MKIII to have a SIMILAR IDLE PROBLEM. I had a blown head gasket in 2009 with 130,000 miles. I had it repaired by the Toyota Dealer in Plano, TX. About 600 miles later, it began idling rough and I stopped by my Firestone Service center and they quickly found the problem was due to a LOOSE connection on the EGR vacuum supply line because of a LOOSE BOLT. I can't prove it, but suspected the Toyota Tech did not tighten it properly which caused it to begin losing vacuum during the 600 miles since the major repair of the BHG.

P.S. In case you never heard of our fair city, you may have heard Toyota is moving it's US headquarters to Plano, TX in 2015!