Hi all, Im posting cause im lost and dont know where to look next. The truck is a 1989 f-150 with a 5.8l 2wd. It is a friend of mines parents truck and was brought to me for a rough idle. It is tough to describe what it is doing cause it is doing several things. First the motor starts fine then idles up then calms down to a laboring idle then idles fine. If you hold the RPMS about a 1000 rpms then it will rise and fall about 200 rpms gently. I have blocked off the EGR valve and have tried every thing that I can think of to figure this out but no such luck. The IAC when unplugged instanly killed the engine, the MAP made it run rougher when unplugged and when you pull a vacum line it dosent run much rougher then it did before, but still pulls good vacum. The truck runs fine down the road. Does have a slight dead spot once in a while when you punch it from a idle. It also back fired once thorugh the TB when I punched it from a idle. The back fire made me think that the timing was off but it was not out of time.

Thanks in advance
Shawn

 

Check the vacuum canister(juice can) on the passenger inside fender, the bottom rusts out of them all the time. And if you haven't done so already pull the codes, just put a jumper between STI and Sig-Rtn lines in the EEC test harness, turn the key to run, and the codes will flash on the check engine light. See Ford Fuel Injection for pics of the EEC Test connector and more info on the codes.

 

The truck was throwing the EGR code which lead me in a round about story to figure out that the EGR tube feeding the valve had broke in half and was why it was throwing that code. The owner didnt really want to put to much money into the truck so it was removed and blocked off.

 

ttt bump for ideas

 

I checked the FPR at idle it stays at 34 PSI and when revved it goes to 40PSI. The repair manual says at idle it should be at 30PSI, is it over fueling it at idle? When I removed the vacuum line it jumped to 42PSI and the motor continued to run rough.

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Getting tired of this truck being in my driveway.

 

You can try removing the IAC and cleaning the crap out of it. Spray inside of it with a cleaner that doesnt affect rubber. Once clean reinstall it and see if idle improves any. If it does then replace the IAC. The majority of bad idle probs on 5.0 and 5.8's are caused by a worn out IAC.

 

Thanks I will give that a whirl if it ever quits raining.

 

Suspect any and all of the vacuum lines and as mentioned before the vacuum reservoir(s). You may be pulling a decent idle vacuum, but trust me you can and probably are sucking a lot of unmetered air too. I found most of the vacuum circuits on my recently acquired '92 F350 to have leaks and would not hold a vacuum when tested individually. After replacing the lines and a new coffee can style reservoir the idle quality has returned to like new condition.

 

I just pulled the IAC off and it looks really clean plunger is not stuck. I dont have a vacuum gauge but the finger gauge seems to read good. AHHHHHHHH to many vacuum lines. I just read that 18 page monster of a post it seems to be pretty close to my problem. How do you test the IAC?

 

I just put the IAC back on. The truck idled better but there is more air coming out of the check valve on the passenger side above the juice can. This has happened on other time but the solenoid going from the charcoal canister to the TB was audible clicking and the air coming from the check valve was pulsing and was also audible. There was more air coming out then when the truck was idling roughly.

I also noticed that the temp gauge was running pretty cool, so I called the owner and she said that it has always done that. Im wondering if it is causing it to stay in open loop, but it ran fine then.

 

bump bump ttt