Fox newbie with an engine question

[andrewVA]

Hi all.
I recently picked up an 89 LX 5.0 coupe to play around with, and being more attuned to the vintage mustangs, I had a beginner question on the 5.0 HO engine.

The car experiences a searching idle for lack of a better term. The first time I start it, it dies right away unless I give it gas. After that, it wont die, but it will drop to about 200 rpm and sputter. The ECM overcompensates and surges the car up to 1500-2000 rpm and then it drops back down. Occasionally, it will idle nice and smooth at about 800, other times it will idle smoothly at a higher 1300.

From what I read, this engine should idle at 672 rpm? I also did a search and found dirty TBs to often be the culprit, so I picked up some cleaner and cleaned out the TB and valve, although I didn't take off the 4 retaining bolts, since coolant started to leak out.
The TB was black with carbon deposits, and I can see more buildup past the butterfly valve.

1) Could this still be a TB problem?
2) Does it sound more indicative of a vacuum leak somewhere?
3) Is there really a coolant passage that runs between the TB and the Upper intake, or do I have a larger problem here?

I appreciate any help you can give me. Currently, the only book I own that deals with the 5.0L engine is the Haynes manual for Fox mustangs. Anyone have recommendations for a good book regarding tuning and maintaining the 5.0 HO?

 

[Dbeck002]

1) Could this still be a TB problem?
2) Does it sound more indicative of a vacuum leak somewhere?
3) Is there really a coolant passage that runs between the TB and the Upper intake, or do I have a larger problem here?

1.

Throttle bodies themselves generally dont cause these problem, but there is a small black plastic housing on top of the TB that is called the TPS or throttle position sensor. You must probe the green wire with a digital voltmeter (ground the meter to block or a good ground) to make sure it is reading below 1 volt, ideally 0.96 volts. If it is not, you must get an allen key (i forgot what size) and play with the throttle plate stopper until the TPS reads 0.96v, generally a very tiny variation in throttle plate angle will cause the TPS to pass 1v which will hurt idle.

Also, the throttle body has a small cylinderical assembly sticking out of its side, that is called the IAC or idle air control module. It controls the idle and most of the time, unbolting it (there are 2 bolts) and cleaning it with a lot of carb cleaner helps tremendously.

2.

It is very possible you have a vacuum leak. Check all hoses, everwhere. If you cant find a vacuum leak but you hear a whistle, to verify there is a leak you will need to spray carb cleaner over the questionable area, if the idle surges than it means the vacuum leak sucked in the carb cleaner and you found the spot. Try this method if you suspect vacuum leak.

3.

There is coolant running through the upper intake and throttle body, that is called the EGR or exhaust gas recirculation system. That is normal, but since you removed the TB you should replace all TB gaskets because they are probably cracking and moldy.

Hope this helps.

 

[srothfuss]

You can also see JRICHKER's post on surging idle. I believe it is at the top of the 5.0 Tech page listed as a sticky. If you have any questions about surging idle or trouble codes (they are free and easy to check with a 87-93 Mustang) he is the guy to ask.

 

[andrewVA]

Thanks for the great suggestions. I dont know how I missed seeing that sticky up there, or getting a hit on it in the search.
There is enough good info in there to hopefully cure my problem, starting with the IAC. I'll make sure to add what worked for me when I get it fixed.

 

[86bluecobra]

my 302 was doing that in my pick-up. i had to clean the butter fly valves of the tb they had never been cleaned. then i removed my iac and cleaned it. now i no longer have the surging problem.