Hi guys,

So last year I was having a problem with the truck (4.6L) surging at idle. As was expected when I cleaned the TB it was fouled. I also did the other routine things at the same time (Cleaned the MAF, Replaced TPS)

So it's' still surging at idle. Here is a more complete description.

At idle (a stop light), in gear (under load) the truck will idle down to about 600 RPM, then it will surge up to 900-1000. It is very predictable. It's worse in cold weather (like now).

At idle, in neutral, the surge is from 600 RPM to about 700 RPM. Far less intense.

From what I've read here I think it's likely to be a vacumn line problem, and most likely the one coming off the back of the TB. Can someone give me some tips on how to track this one down (or point to a good thread?).

Lastly after I cleaned the TB the problem toned down for a while but did not go away, if it's vacumn lines it might be explained by me taking them lose and then hooking them up again.

Thanks for any pointers.

 

i'd clean or replace the iac. its on the back of the 90 tb elbo. i need to clean my iac soon because mine kinda acts up in park and neutral.

 

Good suggestion.... but I forgot to add a couple of things.

1. I changed the IAC last year.
2. I also cleaned the MAF sensor.

But I have new info.

Yesterday I decided to get my plugs changed and went to the dealer to see how much it would be. It ended up being $135 after the "saturday discount" (read: slow business on Saturday). I had them look for a vacumn leak also.

When I picked the truck up it ran perfectly smooth, no surging, I kept waiting for it all the way home (20 miles) but not surging. Then I let the truck sit for a couple of hours and went to run an errand and it started SURGING again????

So I'm not sure excactly what's going on but it's frustrating.

The maintenance yesterday included all the plugs, and one new coil, and checking for vacumn leaks.

Any suggestions?

 

It would not be unheard of for a newer part to fail.. I would pull the IAC off and take a look at it see if its gunked up. Clean it up. Did you get the Ford IAC? I've been getting them at autozone with no problems so far.

 

Ok,

More data.

I decided that 100k on the fuel filter was probably enough and that it could be contributng to the surge problems. So I swapped it out for a new one.

When I did the surging stopped, for 2 1/2 days? Then it came back.

For 2 1/2 days no problem, no surge, and I had declared the problem solved. But now it's back? So I'm not sure what to do.

I could certainly clean the IAC again, and I may but I think I've noticed something else. I dawned on me that maybe the only reason I can detect the surging is because when it's at idle it's obvious. Driving home the other day on a hill I kept my foot very steady on gas and watched the RPMs and listened to the engine and although the RPMs were steady I swear I could hear the engine surging under load going up the hill. The sound was as regular as what I get at a full stop it's just that the RPMs didn't go up?

So:

1. Is that possible
2. If so what might cause that?
3. Maybe I just need more drink more and it will all go away.

Thanks for any help.

 

I am interested in your finding, had an 87 Crown Vic 5 liter, sold for it salvage without me ever finding the answer

 

Get a fuel pressure gauge and test the fuel system. Running a fuel filter that long can cause other fuel system parts to fail mostly the pump it has to work to hard to maintain pressure.

 

When you did the plugs did you get new COPS aswell, my truck had a problem like that and new plugs along with cops fixed it.