tdcarter72

My truck skips/surges under any touch of the gas pedal. Kind of hard to explain, but when I give it gas, it acts like it is running out of gas then picks back up. It don't do it as bad at cruising speed until I need to get back into the gas. I replaced the fuel filter about 3 weeks ago.

Thinking it might be a fuel problem, I run a pressure test today and here are the results.

1987 F-150 302 EFI (duel tanks)

Engine Off - 38 psi
Engine Running (vacuum hose on) - 32 psi
Engine Running (vacuum hose off) - 42 psi

Turned the truck off and let it set and the pressure dropped from 38 psi to 32 psi after 5 minutes and dropped to 26 psi after about 30 minutes. After an hour the fuel pressure was at 22 psi.

If I have read the manual right, these numbers are good and my problem is something else.

I guess I could have a problem with plugs or plug wires. They were replaced about 3 months ago.

Anybody have any ideas?

Just call me Sean

iTrader: (1)

Have you tested the TPS? Did you check for codes? I know you've posted before and I prefer not to have to go through 10 pages of an old thread so I like how you started a new one but refresh our minds with that info.

1987 F-150 XLT 4x4

It could be:
Cracked Distributor Cap
Old/bad gasoline
Fouled Spark Plug
Defective Spark Plug Wire
Vacuum Leak

Pour a bottle of Seafoam Motor Treatment into the gas tank that you know has good/fresh gas in it, it cant hurt.

All I got.

94Bronc

fuel filter, timing, dirty injectors...run your codes, swap your fuel filter if it hasnt been done recently, check your timing, been a while since last tune up?

ymeski56

Tested both tanks?

More than 5PSI within 60 seconds after pump shutdown would n/g.

Catch us up. IAC cleaned & tested? Codes?

tdcarter72

Ok, like Sean mention, this is an on going issue. When I first started on this project (other threads). I started with pulling codes, after getting all the codes taken care of with the exception of a low idle code, I am still left with this skip/surge issue (which I hope will go away when this is fix).

I have replace the following on the truck along the way of clearing the codes: TPS, EGR Valve, EGR Regulator, IAC, All new Vacuum Hoses. I have also changed the fuel filter (3 weeks ago), plugs, wires, rotor cap and button (3 months ago). I did a seafoam treatment last weekend and run a bottle in the gas tank.

Today I ran the fuel pressure test and checked the timing. The timing was dead on 10 BTC when the truck was idling right.

When I get to where I can stand to look at it again , I am going to pull the plugs and wires and inspect them, might just change the plugs while I have them out. After a test drive, I am going to pull the codes just in case something else has come up.

I really don't know any more to do.

mgranata

If it is a gentle surge look towards a fuel system problem. If the surge is sharp and abrupt look towards an ignition problem. One very quick check for weak ignition is to load the motor by doing the following: (1) with the motor running and auto trans in drive put one foot on the brake pedal (2) while holding the brake firmly press slightly on the accelerator pedal with the other foot, hold the break hard enough to prevent the truck from rolling and rear tires from spinning. This creates more demand on the ignition system and weak components will cause the motor to have sharp abrupt skips (missfires). Doing this with the motor cold creates an even larger demand on the ignition system. Keep in mind though, a bad TPS will also cause sharp abrupt surges. But in my experience, a bad TPS was more common in the late 1980's when fuel injection first appeared. Also, a bad TPS typically acts up at only one throttle position, the throttle position where the vehicle gets most of its use. I have only checked fuel pressure on GM fuel injected vehicles. My 1995 Ford service manual specs the pressure as 30-45 psi with the engine running and hose connected, and 35-45 psi with the key-on engine-off condition. Your fuel pressure is at the low end of the scale with the engine running hose connected condition. Does this driveability condition exist with either fuel tank? Are these the original fuel pumps? What type of milage is on the vehicle? With the gas cap removed and engine running, listen to the pump at the fuel filler inlet (where the gas cap is). The sound of an electric fuel pump is a pretty good indicator of its condition.

tdcarter72

It did not cross my mind to do the fuel pressure test with both tanks. I do know there is not a proformance differance when using either tank.

It is a sharp and abrupt surge when under a load, it is a gentle surge at idle. One thing about this is at a start up idle it wants to do dead, once the truck warms up it idles alright with the gentle skip or surge.

mgranata

Double check for vacuum leaks but it sounds ignition related because a cold motor puts greater demand on the ignition system. When was it last tuned up? It may be ready for a major tune up. I know the symptoms you describe also occur when the distributor rotor and/or inside of the ditributor cap are dirty. And both of these should be replaced during as part of a tune up. Like one of the previous posts mentioned, I would also poor a bottle of fuel injector cleaner in the gas tank. At the least, it is preventative maintenance.

Just call me Sean

iTrader: (1)

The fuel pressure is barely low, only 3 psi. The plug wires have to be routed a very certain way on V8s to prevent crossfiring.