mark-hatfield@comcas

Looking for advice on how to smooth out idle on my stock automatic 86 (cast iron heads), 192,000 miles! At warmed up idle wants to hold at 600 but starts looping between 400 and 800 and sometimes shuts down.

Here's my data:

IAC set in drive to 450
TPS set to 54
EGR holds vacuum
No obvious vaccum leaks
Timing set to 6 BTDC with connector unplugged

Fuel pressure at ignition on -- 20 psi, engine on -- 38 psi, engine shut down holds between 15-20 for over an hour. Gas tank maintains pressure.

Injector ohms by cylinder# as follow (they are black with no noticable colors or marks):
1 -- 17.5 loud tick
2 -- 17.7 loud tick
3 -- 17.4 loud tick
4 -- 17.7
5 -- 17.4
6 -- 17.3
7 -- 17.4
8 -- 17.2

Experiences some brief valve lash/rattle when I step on it and get the RPMs up quickly.

Slight exhaust leak on passenger side of engine compartment (became apparent while Sea Foaming the top through the brake booster line).

Things I've replaced over the last year or so for a variety of codes and issues:

new EGR solonoid (found at Carlisle for $10, why not)
new MAF and relays
new ECM with old stock chip
new 02 sensor
new knock sensor
new fuel filter

Dash board stats while cruising:

Voltage to alternator -- 13.4
Oil Pressure -- 12
Coolant -- 195 degree thermostat -- 190-215 degrees
Oil temp -- 225 degrees

Thinking about bumping the timing but thought I'd check with the board to see if anything jumps out at you that I should tackle other than the exhaust leak and the valve adjustment.

The car is running better now that it has since I started playing with it a few years ago. I feel I've come so close to getting it right on.

Thanks for any advice.

Mark

Bronze85

Oil pressure is only 12 psi at cruise? I would confirm the true oil pressure before proceeding to an idle issue.

jsinga

Idle hunting is most often casued by a vacuum leak. start plugging hoses and driving it.
Look at the duct between the MAF sensor and throttle body.
Don't forget the brake boster, my 87 got a slight split in the diaphram a few years ago.
If you think it's a sensor problem try unplugging them one by one. Most of the time it will still run. The ECM will just midrange the sensor value.

JS

leesvet

A couple things...

air is most often why idle roams or wanders. Once in awhile injectors that are leaking down can contribute.

The IAC can be making appropiate steps and reactions but that does not mean that the pintle is seating. They get horribly gummed up with carbon and will stick..

If there is a coolant bypass on the throttle body there might be a vac leak on the underside. Thats where the water used to stay but it also has air passages in there...

Butterflys worn, leaking or the shaft. Check for shaft end-play. Just another possible source of unmetered air.

With close to 200K, you really NEED to call Jon...Those stock injectors were junk when they were new. They don;t get any better. Multec made Bosch wealthy.
Go to youTube and search for "fuel injector" and watch some of the FIC videos and other info. Its very informative and you'll want to get new ones the next day.
Its the best $250 you'll spend next to an all aluminum radiator !

mark-hatfield@comcas

Oil pressure is only 12 psi at cruise? I would confirm the true oil pressure before proceeding to an idle issue. Thanks Bronze, I checked the oil level and I was almost a quart low, filled it up and took it for a drive. Oil pressure is at 12 on the low end (A/C on, in drive sitting at a stop light), cruisng I could get it as high as 40 psi. Is that within spec?

Idle hunting is most often casued by a vacuum leak. start plugging hoses and driving it.
Look at the duct between the MAF sensor and throttle body.
Don't forget the brake boster, my 87 got a slight split in the diaphram a few years ago.
If you think it's a sensor problem try unplugging them one by one. Most of the time it will still run. The ECM will just midrange the sensor value.
Thanks JS -- I actually had to replace the brake booster a few months ago, I'll do the vacuum pluging and sensor disconnects to see if something jumps out.

The IAC can be making appropiate steps and reactions but that does not mean that the pintle is seating. They get horribly gummed up with carbon and will stick..

If there is a coolant bypass on the throttle body there might be a vac leak on the underside. Thats where the water used to stay but it also has air passages in there...
Thanks LesVet -- the IAC is brand new, the old one had come apart inside the TB, the TB is bypassed due to a water leak in the TB, I'll check that suggestion out. I may be in for a new TB with the new injectors, I will definately use Jon, I've seen his videos and was impressed.

Bronze85

The pressures are low but considering the mileage.......Have you done this procedure? http://forums.corvetteforum.com/c4-t...dle-speed.html

mark-hatfield@comcas

Done this procedure many times, most recently yesterday. The IAC and TPS are spot on.

Where should my oil pressure be? Ideas why it's low. I did have a leaky oil pressure sensor that I replaced a few weeks ago, leak has stopped.

Thanks,

Mark

leesvet

These are very sensitive to poor connections and resistance. The harness can also get wire rot in that area and give odd readings. Does the oil press start at 40+ when cold and go down as it gets warmer?

Your Oil TEMP bothers me a bit. Being that much hotter than the engine coolant after a normal drive tells me that the oil is working harder. Its being slipped out thru bearings and that can cause it to get hotter as its squirted out under pressure thru a small opening. Thats how it gets hot and stays hot.
That needs to be investigated.

jan-erik

Hi.
I have an 1985 z-51 4+3. I had the hunting idle and it was vacuum leak and a bad EGR. This is ok with your 86 as I understand.
The fuel pressure is very low when ignition on. The pressure should be about 39 psi. When engine idles the pressure decreases about 5 psi. The hold of the pressure seems short but will give you no problems when driving.
It might be the fuel pressure regulator giving you the low pressure when ignition on.
You said you had an exhaust leakage. That might give you faulty o2 readings if the leakage is in front of the O2 sensor as it seeps air into the exhaust system.

AGENT 86

Try increasing your minimum idle and then reset your TPS.

aklim

I'd most certainly hook up a mechanical gauge to see what the oil pressure is doing. You can hook it up to a port at the top of the valley or at the oil filter area, there is another port.

aklim

How about checking if the IAC passages in the TB are gummed up or dirty?

Another trick is to use a propane torch. With fire off and gas flowing, pull of the connectors to the fan but be careful not to overheat it. Run the tip of the torch along the air hoses and items to see if it changes RPM.

aklim

Assuming the IAC is working, I would like to hook up a scanner to see what the ECM is trying to do. It would be in the IAC counts section though. Ideally it should be around 15-20 but if it is at 0, you might be out of adjustment or have an air leak. If it is above 20 you might have to open up the throttle body blades a little more.

aklim

Won't the ECM command an idle and if you increase it, it will try to control it by closing the IAC and maybe adjusting the timing and fuel to keep with what the ECM Command Idle is set at?

Could it be that the Command idle is wrong for this application?

AGENT 86

It would lower his IAC count and hopefully the ECM would have an easier time controlling idle.

mark-hatfield@comcas

40 is the highest I have seen the oil pressure, usuallt after warmed up and under throttle. At start up usually around 20, at idle as low as 12. The 225 oil temp is the highest I've seen it, that was yesterday, air temp was around 75. Today was around 55 outside and oil temp stayed between 190-210.

mark-hatfield@comcas

Is there a test for the FPR? (I checked the top port for vacuum and it holds) As far as the O2 sensor, there is only one and it is on the drivers side just off the manifold, the leak is on the passenger side before the pipes come together under the car. Would this possibly still be affecting the idle?

mark-hatfield@comcas

Increased the minimum idle using the correct procedure reset the TPS as you suggested, helped a little bit but still hunting between 500-800, did not stall out on my last run!

The short hose that connects the TB to the crankcase tube on the passenger side was frayed/cracked on the ends so I put a new one on, it also helped some.

Still not where I want it.

aklim

You can hook up a fuel pressure gauge to the FPR. IIRC, the stock FPR will hold pressure for a bit after the pump is turned off.

aklim

Did you clean out the TB and the IAC housing? Every couple of years, I would take the thing out and remove the housing for cleaning. I spray cleaner all over till it is spotless.

Use the propane torch around the housing and see if it is leaking air in there.