Holden

Sorry for crossposting this, but I'm signed up for a HPDE on Monday and need to get this resolved. Here's what happened:

1) Battery died last week. I think this has nothing to do with the problem. It was 6 years old. Replaced battery and car ran fine.

2) Today, cruising around, the engine died. My J&S Safeguard Classic was not turning on with the ignition, so I looked at it's wiring. I found that the TPS wire was disconnected, but this wire has been disconnected for a while (The safeguard had been reacting to piston slap, now I know why!). I reconnected it, but the J&S still doesn't turn on.

3) So, I bypass the J&S to get the ignition signal to the distributor. Engine starts, but is in some kind of super-limp-mode. Idle is at 3500rpm. Check engine light is on. I limp home.

4) I disconnect the battery to clear any codes. I figure the ECU lost it's connection to the distributor, so any codes would be pretty meaningless. (i.e. Ignition failure)

5) Hours later I work on the problem. It turns out that the J&S ground wire had come loose from it's grounding point. I fix this and the J&S is working again and the engine starts without the J&S bypassed.

6) However, idle is rapidly (like twice per second) fluctuating between 1800 and 2300rpm. Engine is cold. No check engine light. It does this whether or not the Safeguard is connected.

What the hell is causing this? Did the ECU dump so much fuel in limp mode with 320cc injectors that my manifold is filled with gas?

Could I have fried the ECU just by having the Safeguard turn off?

Much thanks for any help. I don't want to drive a rental on Monday at Lime Rock Park.

dustin

Does the car drive OK? Dead map sensor wiring will do exactly that... (A long time ago when I had a V-AFC, the +12v came loose and that is how the car acted. Had no CEL for some reason)

Holden

I didn't test drive it. I figure it'll never pass tech inspection for road racing idling like that so why bother?

My VAFC is working fine as far as I can tell. My next step is yanking the ECU and going through the rat's nest of wires splicing this and tapping that. I will zero in on the MAP sensor wire. Thanks for the clue.

Holden

According to the VAFC, the MAP sensor is sending 2.75 volts with the engine not running. My kids are sleeping, so I can't start it up again tonight.

Is 2.75 volts correct for 0" vacuum?

falcongsr

i get 2.85v in dallas elevation of 1000ft above sea level

Holden

Thanks. I'm at a few hundred feet. So the MAP sensor is good (as expected) and the signal makes it to the VAFC. I'll check the VAFC to ECU circuit tomorrow.

Holden

Update:

- Car drives fine. Boost is great. VTEC activates. BTC retards properly.
- MAP signal is getting to the ECU correctly.
- I can't find any wiring problems or any malfunctioning components.
- ECU is not throwing any codes.

- STILL, idle is crazy. When warm it fluctuates between 1000 and 1800rpm faster than once per second.

How does a car drive when the Idle Air Control valve goes bad?

Any ideas?

Thanks.

Holden

Another update:

- J&S Safeguard, JR BTC and VAFC all completely removed. Wiring at ECU looks fine.
- Idle still fluctuates.
- Plugging hole to IAC valve in throttle body makes the idle bob slower, but it still bobs up and down.
- For first 10-15 seconds after starting, engine idles steady at 2000rpm (normal start idle). Then it starts going up and down.
- SC Bypass valve is OK.
- No vacuum leaks.
- I'm getting pissed.

dustin

Unplug IACV and use the idle set screw to control idle. Does it idle OK then?

Does your car have a fast idle valve under the throttle body?

Holden

Problem solved.

When the engine died yesterday, it burped boost back toward the throttle body. This blew off a vacuum cap on a 1/2" ID vacuum tee that is part of the oil/gas vapor recovery system. This tee is buried underneath the blower.

A 1/2" vacuum leak makes for a pretty violent idle occilation!

Thanks for your help. I could've sworn this was an electrical problem.

I'm off to the races!!! My brother came through and offered his 240SX/SR20DET for the event, so I had some insurance. Thanks Bro!