I'll ohm out the coils on the new ISC to see if they're still OK. They were reading 40 ohms prior to installation. Then I'll see if putting in my spare ECU makes a difference.



Temp gauge reads normal after warmup--about 1/4 above the lower limit.



Haven't messed with the speed adjusting screw yet--since the idle is so far out of range I figure there's an underlying cause that should be fixed first.



Crud is a possiblity. Is there a safe cleaning agent that could be used on the FIAV? I know solvent like carb cleaner isn't recommended, but maybe electrical contact cleaner or something. Worth a try.



I'll do that prior to swapping out the TB.



I actually already installed the new ISC. I don't know that the old one is bad. The coils were still in spec, but the pintle was a lot looser than the new one. I feel safer with the new one because the higher resistance coils are easier on the ECU drivers. But in any case I'll check the new coils again.

Thanks for the advice gents. I'll also do the curb idle speed inspection before I dig into the TB (don't have a tach-box yet). That includes a timing check, which is also a possiblity I suppose.

I have a car-part TB on its way. After pricing out a new one ($850 <img src="/forums/images/graemlins/zombie.gif" alt="" />) it will give me peace of mind having a spare even if that's not eventually what it turns out to be.

Taking a break from wrenching.

Looks like I had/have multiple contributing issues going on.

1. Bad ECU
2. Idle position switch not adjusted right
3. Timing.
4. Something else?

As I was exiting the parking structure at work yesterday, my ECU died. Coasted into open parking spot and noticed the telltale fishy smell. While waiting for a kind coworker to give me a ride to pick up my spare ECU I searched +idle +ECU and found a few cases of ECU's going bad causing idling issues. Great, I thought at least the cause is found.

Not so fast. Checked new ISC coils and they are still in spec at 40ohms. When I installed my spare ECU, the idle was back up to 1800 cold or hot. Found that the idle position switch at pedal release position was open. Adjusted the TPS according to the FSM, and verified that the switch was closed (continuity) with the throttle lever touching the fixed SAS, and open (no continuity) when the throttle lever is opened a bit.

Idle seemed to operate more normally, with an exception. 1800 at first startup with the engine cold, then after a minute the idle bounced around for awhile, down to 1300, up to 1700, repeat, repeat, repeat, until it warmed up more, then dropping down to a steady 1000. Flaky fast idle air valve?

Did the curb idle speed check. Found my timing with the timing terminal grounded was 0?BTDC. Adjusted timing to 5?BTDC. Removed the ground from the timing terminal and the actual timing jumped to 23?BTDC instead of the expected 15?BTDC. I didn't check actual timing prior to adjusting the distributor. Idled at 1500rpm.

Adjusted the timing to 3?BTDC with the timing terminal grounded (right on the edge of the spec) and the actual timing with the ground removed is 18-15?BTDC and 1200rpm idle.

That seemed close enough to try adjusting the basic idle speed. Grounded the timing terminal, and grounded pin 10 of the self-diagnosis connector. Found that the speed adjusting screw on the throttle body was already all the way in, so I can't get the idle any lower than 1200.

Next, I'll replace the throttle body.

Any concerns about the way I set the timing?

I think the compuer requires a number of starts to adjust itself.

I think the proceedure was to let warm up then turn it off, then start it right back up and wait till idle stabilizes then off ...and repeat 3 times or more.

In other words, it takes awhile to adjust. Give it time

---------

that's for idle speed not ignition timing, and I might be confusing this with some other vehicle but it is worth a try.

Only ground the one under the hood.


I would back the screw out half way and run it while (off- on proceedure)to let the computer get used to that... then try adjusting the screw.

Also make sure all grounds are good.

Changed the throttle body and it's working the same as the original one. Initially, it was idling at 1500, and turning the speed adjusting screw all the way in (like the original) brought it to 1200.

I did find why I was having that surging issue. Apparently it was the new ISC; replacing the new ISC with the old one eliminated the surge. So now my idle starts at 1800 cold, and gradually goes down to 1200.

Summary of work:
1. Verified ISC is good.
2. Replaced engine coolant temp sensor
3. Adjusted and tested TPS/idle position switch according to FSM.
4. Verified ECU good.
5. Adjusted timing.
6. Verified throttle body good.

My last idea is to test all the MPI component harness connections to the ECU for shorts to ground or opens.

You've checked about everything else, so:

1) Check all vacuum lines for leaks.

2) If you can find a MAF sensor that you know is working correctly, try swapping it. There is a resistance test for the MAF, but I don't think it checks all of the MAF functions.

3) Check the wiring harness and connections to the MAF - "lordtrunks" was having a similar issue and just PM'd me that he found the problem - someone had hacked the harness and left bad connections.

4) Cycle the MAF connector a few times and/or use some electrical contact cleaner spray on it.

Frank

I checked all the vacuum lines and they are in great shape.

Replaced the MAF with a spare (actually the original, been running with the spare since I found it), no change. The MAF connector pins look tight. Cycled it a few times.

The 1990 FSM has very comprehensive tests for the harnesses. Checked the ones to the MAF, ISC, TPS, engine coolant temp sensor, inhibitor switch, power steering oil pressure switch (tested the switch operation also). Results normal.

Turned on the air conditioner while idling, and the idle bumped up 100 rpm or so, so that seems to be working okay.

Figured it would be worthwhile to check the components on the vacuum line, so I tested the purge control solenoid valve with a hand vacuum pump w/vacuum gauge and that's working normally. The PCV valve tested okay. Connecting the vacuum gauge to the nipple on the throttle body that the purge control solenoid vacuum line connects to drew a steady 19psi vacuum.

Running out of logical ideas, I tested the crank angle/TDC sensor harness and its fine.

About the only thing I haven't messed with is the fixed SAS, but since I swapped the throttle body with no change, I don't see how that could be the problem.

I'm wondering if maybe the fuel pressure regulator is acting up; it's got a vacuum line connected to it?

I have noticed something unusual. This reappeared. I thought putting in the original ISC corrected that, but it happened again today when warming up from a cold start. The bouncing rpm's also occasionally happens when I'm driving. It's most pronounced when I'm driving down a slope, foot off the accelerator, in Drive, coasting at 40mph. The engine is at 1700rpm, then drops to 1200 quickly, then back up to 1700 and so on. When this happens, my Autometer vacuum gauge shows about 20psi when the engine's at 1700, and drops to 17psi at 1200. Strange.

This thing's got me stumped. <img src="/forums/images/graemlins/confused.gif" alt="" /> I gotta get this figured out; while I can live with the high idle in 2H, when I drive in 4LO with these Aussie gears, it bucks like a bronco when putting it in gear. Hard to hold it back with the brakes in 4LO also.

1) Have you polled the ECU with an analog meter to check for stored codes? If not, do that next.

2) How old is the o2 sensor? The system goes into closed loop after warmup and may be throwing a bad voltage. You can monitor it with a meter, but if it's old it should be replaced on principle.

3) Is the ECT sensor ok? It's also used in closed loop calculations. You can test with a meter.

4) This is a long shot, but possible - since you say the issue is worse (or only) at/near idle, the distributor crank angle sensor could be bad. The distributor is an optical reader with 2 sets of slots. Below ~1000 rpms or so, the ECU uses the fine set of slots for idle control ignition advance adjustment - above the cut in/out point, it reverts to the coarse set for ignition control. The easy way to test it is to swap in a known good distributor, but make sure you get a '90/'91 distributor - they may be different from the '89.

5) Mounted on the coil bracket is a small round capacitor and an LC filter that passes a conditioned signal to the tach. I'm not sure what, if anything, happens to ECU calculations if those go bad, but if you can get the engine to idle erratically for testing, try moving those little pigtail wires around and see if anything changes - the wires get brittle with heat and age, so don't get too aggressive. If you have a junk yard nearby, replacing that assembly is one thing to try...... some Diamantes have the same coil assembly and are easier to find than the trucks.

5) You've replaced the ECU, right? If so, that shouldn't be the problem, but you should still poll it for codes.

There's a pony in there somewhere.

Frank

1. The ECU is firing steady short pulses, so it's clean of codes.

2. Replaced the O2 sensor about a year ago. New OEM assy.

3. ECT sensor resistances are good.

4&5. I'll hit up the junkyards for those and give them a shot.

5(#2). Yep, I put in the original ECU a week or so ago when my spare that I was running shot it's caps. The original isn't actually the original, but one that was replaced by a shop about 7 years ago. I don't know if the shop put in one that had the caps replaced or not, but I recall they charged me about $400, so I hope it was rebuilt with good caps. I'm sending the fried spare to AvPro for repair and I'll try that out when it returns to see if there's any change with the idling.

Thanks for your help, Frank! <img src="/forums/images/graemlins/cheers.gif" alt="" />

Tested the fuel pressure and it's 44psi at idle and 52psi with the fuel regulator vacuum hose disconnected. FSM says it should be 38 at idle, and 47-53psi with the hose disconnected. That's 6psi higher than it's supposed to be at idle--higher pressure, more fuel past the injectors, higher idle. Sounds logical to me anyway.

The system holds pressure well; didn't drop a bit three minutes after shutting off the engine.

Suspects are the fuel pressure regulator and the fuel return line. However, since there is a difference in pressures with the vacuum hose connected and disconnected, it seems to me like the return line can't be blocked. But maybe it's restricted somehow. As far as I can tell, there isn't anything in the return line other than hoses and hard lines back to the tank. Is there a test for restriction, or should I just go ahead and replace the fuel pressure regulator?