87 Grand Marquis Idle fluctuating

[87GMQ]

With the car running in park the idle fluctuates. Basically its revving up and down. Nothing crazy but it also does it in drive too. I can see the lights dim and then brighten. Alt charging at 14.5 at idle and has the 3g swap.

Any ideas?

[87GrandMarq]

Does it coincide with the ac compressor cycling? Check for vacuum leaks and dump codes to cover all your bases.

[gadget73]

sticky IAC, bad air or water temperature sensors, and vacuum leaks are common causes of this issue. Check the vac lines first, cheapest and the most likely if you haven't replaced them already. The line to the MAP especially must be good.

[DerekTheGreat]

Yeah, you can try & unplug the IAC to see what happens. Throttle body and IAC are probably all gummy and need a cleaning. I did the 3G swap to my car too. I never see the lights dim unless the engine dips below idle speed.

[gadget73]

yeah they charge unless the engine is below about 500 rpm. The old 2g doesn't charge much below about 1000.

[87GMQ]

Tried cleaning the Iac and Throttle body, They were not dirty and it still has the same issue. Map sensor vacuum line is really soft. (what size is it and where can I buy it)

Also unplugged the iac while running and the car shuts off.

Any ideas for the next step?

[gadget73]

I want to say the MAP line is bigger at one end than the other. 5/16 at the manifold and 1/4 at the sensor maybe? Kind of a deal where you have to measure it, and probably use one of those reducer fittings. Parts stores have vacuum line and assortments of plastic connectors and reducers. The upper intake has to come off to get to it. Check or replace all of the lines under there, the one going out to the evap solenoid also tends to get mushy and the plastic lines crack.

ohm check the temperature sensors while you're at it

http://www.grandmarq.net/oldfuelinjection/page28.html

http://www.grandmarq.net/oldfuelinjection/page29.html

coolant sensor is in the heater hose fitting next to the upper rad hose connection, air temp is in the lower manifold between #5 and #6 injector.

[87GMQ]

Replaced the map hose today and the pcv hose, changed the fuel pressure regualtor because it had tiny bit of fuel in the vacuum line.

Still flucuates in park, but I could only get it to do it once in drive. Seems like its getting better, But I wouldnt call it fixed until it stops flucuating in park. Making progress though.

[sly]

If you didn't take the battery cable off during all of this, try that. The ECM may need to relearn idle (clear the learned stuff from the old sensors).

[87GMQ]

If you didn't take the battery cable off during all of this, try that. The ECM may need to relearn idle (clear the learned stuff from the old sensors).

Good idea, i'll try that.

[friskyfrankie]

Tried cleaning the Iac and Throttle body, They were not dirty and it still has the same issue. Map sensor vacuum line is really soft. (what size is it and where can I buy it)

Also unplugged the iac while running and the car shuts off.

Any ideas for the next step?

The size of the MAP Vac line was??????

[87GMQ]

The size of the MAP Vac line was??????

Dont remember, everything I used today was from this: https://www.amazon.com/gp/aw/d/B0BKK...b_b_asin_image

[friskyfrankie]

Dont remember, everything I used today was from this: https://www.amazon.com/gp/aw/d/B0BKK...b_b_asin_image

Added it to my Amazon Wish List so thanks for the link as a friend has a '88!

[VicCrownVic]

What I recall from almost a decade ago was the map sensor vacuum port and the port on (under!) the upper intake at different sizes. The intake side is much larger. I'm guessing but I think the hose ID for the sensor side is about 1/4 inch while the intake side was more like 3/8.

[friskyfrankie]

What I recall from almost a decade ago was the map sensor vacuum port and the port on (under!) the upper intake at different sizes. The intake side is much larger. I'm guessing but I think the hose ID for the sensor side is about 1/4 inch while the intake side was more like 3/8.

Yeah the vac line on my friend's '88 is somewhat soft but still useable. Just don't feel like pulling the intake as I am not sure of the line's condition there. Like I said - soft at the sensor. Would be nice to know the exact dimensions, at both ends. If needed, I'll test the sensor end with vac lines I already have and could patch a line in as fall back into the intake as I can get but that is not the right way to do it. One of those "down the road maybe projects"!