weird electrical problem

[Grant]

I'm stumped

My 86 2.5l had a no start contition. It was the feul pump not getting power, more specifically, bad ground. I cleaned up the one behind the drivers tail light and used aircraft connection sealer enhancer. Also added a ground just aft of the gas tank where the harness goes all the way back. also used the connection jelly then undercoated over it all.

Now the truck will start and run as long as you foot is on the gas. when you take it off it goen into a 800rpm/0rom/800rpm cycle and eventually stalls.

 

if you turn on the running lights or the headlights, it idles just fine?

 

any Ideas?

[Dechion]

Engine block ground maybe? sounds bizzare.

 

I would just check every ground I could find for starters.

[Grant]

I'm going to ground the cab to frame, pu body to frame, block to frame, and frame to battery. All new ones. We'll see what that does.

[Grant]

i have cleande the block to cab ground. Made a ground from the fender to the battery. so with 4 grounds all good and clean it still does the weird idle except when the running lights or headlights are on?????

[HOrnbrod]

Measure the charging voltage across the battery posts @ idle, then move the - lead to each ground point leaving the + lead on the + battery terminal. Repeat the voltage readings with the headlamps on then let us know. :wrench:

[Eagle]

I'm going to ground the cab to frame, pu body to frame, block to frame, and frame to battery. All new ones. We'll see what that does.

Good luck finding a frame under the cab.

 

You do know what "unibody" means, right?

[Grant]

OK,

voltage 14.75 lights on at engine block, firewall, fender, ground under the cab by fuel pump. ( the one I made )

 

lights off No Change. ?

[HOrnbrod]

OK,

voltage 14.75 lights on at engine block, firewall, fender, ground under the cab by fuel pump. ( the one I made )

 

lights off No Change. ?

 

Well, that about eliminates a grounding problem. I'm not familiar with the 2.5s and don't have the electrical manual for it so I'd just be guessing. Unplug your ECU and check for any recessed and/or corroded connectors. If no joy I'd start checking the sensors. I can't see how the headlamps ON can affect idling if it's a fuel problem.

 

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

[Grant]

anybody know where the ecu hides on an 86 2.6L?

I agree, it doesnt seem like a fuel problem.

I did have a big vacuum leak, the brake booster check valve but that only cured the high idle, not the weird idle!

[JeffN]

Sounds like a power supply problem, the headlight switch sounds like it is somehow backfeeding voltage to the vehicle when it's on.

I do not know anything about that year/engine combination.

I would start by checking your voltages coming into the fuse box with the lights off then turn them on and see if the voltage comes up on the fuses.