1998 Mercury Tracer Rough Idle, New A/C Compressor

I carried out a number of tests using the information posted where I could follow it, remember I’m not a mechanic and not familiar with all the terms presented in it. I also have no way to identify everything it mentions. I know I need to buy a book as it’ll help quite a bit!

Please note I did carry out each test approximately 10 times to ensure accuracy. Tests were carried out where directed with power set to on and when needed with the engine running:

Test 1:
Code 1504 present, previously had 1507 until I replaced the IAC. Same code with both new and used IAC.

Test 2:
Following the direction given I measured the voltage from VPWR to ground at 11.8v more than the 10.5 minimum stated.

Test 3 & 4:
3. Resistance between VPWR and ground at 12.8 ohms, 4. Resistance between IAC terminal and housing over 10,000.
Test 7:
I’ve no clue where the PCM 104 pin connector is in order to disconnect it. Would welcome any direction finding it. I would really like to test that circuit fully to be sure nothing is shorted.

Voltage between Pin 83 (at the harness) and ground is measuring 0v. Resistance between Pin 83 and ground is 203 ohms
test 9:
Have no way to identify test Pin 51 (ground) do not have a breakout box or scan tool. I’d prefer not to buy a scan tool as I already know the code 1504 and before replacing the IAC it was 1507 over speed.

Test 20:
IAC valve replaced, throttle body appears to be fine. Don’t have the over speed code after replacing the IAC. Checked for leaks and had a garage check for leaks none were found. I checked again with carb cleaner and got a minor increase in idle when spraying in the area highlighted in my post above. PCV valve and hose appears to be fine.
Test 21:
I’ve no clue what this is even talking about! I’ve a feeling this was checked at the garage as they ran a test for vacuum leaks and found none after flushing out the A/C.

Test 22:
Started engine let it heat up (actually drove around for a few minutes) parked waited for everything to come to a rest. Disconnected power to IAC. No change might as well be driving without the power connected to the IAC! I do need to find that 104 pin PCM connector?

Test 24:
Need to find Pins 53, 103 have no breakout box. Don’t have a high idle anymore or normal idle, my idle is now mostly low apart from when starting the engine from cold which is expected to be a bit higher than normal.

PCM – Powertrain Control Module, I assume this is the computer? If so I can rule this out, at least the garage did. They replaced it and the replacement responded the same as the old one so swapped back in the old one as it worked fine and sent the other back.

Thanks again for looking at this!

I should also note that this car drove perfect when I bought it. After I had the A/C replaced it shook really bad! Now the shaking isn't too bad but I often wonder could something electrical been nipped, torn or shorted while changing the A/C?

I checked yet again for vacuum leaks as I did find an area giving me a slight increase in idle when sprayed with carb cleaner. I wanted to ensure 100% that nothing was cracked, open or rotted away.

I found see images below. Under the IAC the throttle body. Below that there is a tube that runs down and it's open on the other end. It isn't connected to anything? When I sprayed carb cleaner on it the idle jumps when I closed the tube the idle drops then starts to come back up again very slowly?

Should this tube be connected to something or is it some kind of drainage tube?

The image was taken from the area highlighted by the red arrow, below the throttle body leaning over the battery on the right side of the vehicle.

Thanks again for the response! I think it may well be that vacuum line running out of the throttle body. I found the other end of the line and from what I've been told is connected back to the purge valve? I assume that the black item in the image below? The line looks like it's rotted away over time it's falling apart.

I got a new line and once I have some daylight tomorrow I'll install it and see if that helps. Any advice on installing it would be welcome. I'm not seeing any clamps holding it on I assume it's stuck somehow. The original line looks like it was stuck to the plastic line that runs from the purge valve (black item) in the image below.

I installed the new vacuum line from the bottom of the throttle body back to where the other broken end was. With the engine running, I sprayed the entire area again with carb cleaner and no changes in idle. I also pulled out the IAC power connector and right away the idle dropped very quickly but it adjusted itself before it stalled and settled into a shake once more. Reconnecting the IAC the idle corrected itself again and the shaking was reduced quite a bit.

I've cleared the computer just so I'm starting from fresh and so it'll relearn everything from the start. I'll drive it for a few days and see how it is. Even now it does feel a bit smoother than before although I can still feel the vibrations from the engine inside the car.

Now I've been driving around for a bit and so far so good. The shaking is down to a slight vibration once and a while with a minor surge up then down but only minor. Check engine has not come back on again.

Assuming your correct and a backfire took out the IAC could it have taken out anything else that would or could throw off the idle just a tiny bit? PCV maybe or anything else I should look at.

Many thanks again for your help and to everyone else who helped!

I'll try cleaning the throttle body as suggested, if I can ever figure out how!

Check engine is back on. P1506:

* Engine Vacuum Leak
* Failed IAC Valve

Considdering I've tried a new in the box IAC, Used IAC and the broken IAC. I'm sure enough I have two good IAC's! Which leaves Vacuum leak? I checked again with carb cleaner and I can't find anything!

Any tips, tricks, suggestions, hidden lines not easily visible I could be missing?