Surges at idle

[Fossiljager]

Swapped a 94 4.L w/ A4LD in a 90 B2

I've read loads of thread pertaining to this issue but nothing seems to address the cause of my surge.

I have a few things left to do on my swap i.e. connect the AC and cruise control and maybe that's the culprit but I have an annoying surge at idle that goes away at speed. No CEL or stored codes. I've checked the TPS against one from Advanced auto and that's not the problem, I've installed a larger MAF and intake tube from a 96 Mustang, I've have the same issue before and after the MAF ungrade. In my experience engine surge normally has to do with a vacuum leak but I can seem to find evidence of one.

A weird thing happened yesterday.... the truck was sitting idling when suddenly the RPM went up on it's own to about 1,500 and stayed there for 2 minutes or so, then went back down to a smoother idle... it's like the PCM saw something and tried to fix it on it's own, and still no codes. I started the truck again this morning and the surging idle has returned.

When I first start the truck it revs to about 1500 as it should (running very smooth) then settles down to around 800rpm then begins to surge.

Thanks for any ideas you might have,

Harl

 

[Flip4ford]

fuel pressure?

 

[Fossiljager]

I've installed a new fuel pump, that seems to be doing the job well enough... The surge is less evident the more I run the engine... I'm wondering if the previous owner so neglected the engine that it developed a kinda death rattle just to stay alive. Now that it's had a little TLC it might be mellowing out on it's own. I still think I have a vacuum leak somewhere....

harl

 

[94Gexplorer]

theres a vacuum line for the cruise control you said its not hooked up that would be a problem

 

[Fossiljager]

The only thing not hooked up to the cruise is the electronics... although I do have a vacuum line I know of under the dash that helps run the climate control, that was mangles when I welded new metal to the fire wall... I can only find that fitting at a bone yard...

Harl

 

[Fossiljager]

I think I fixed the problem w/ my 94 4.L surging...

I dumped a 1/2 can of Seafoam in my motor, let it sit for 2 hours and presto.. the engine smoothed out and is running like a top.

Chock one up for Seafoam.

Thanks for your help guys....

Harl