Hey everyone,

I am trying to get my beast (88 f250 5.8L) to run a little better, and I am running into some problems. I wanted to give some symptoms and see what people think might be wrong.

Ok, first off, my vacuum system is screwy. I don't have heater controls (the blender or whatever switches between defrost and the floor). My cruise control dropped out recently (it was working), and my tranny shifts really late and really rough. I put in a vacuum gauge, and it is sort of erratic. Normally, when I first start the beast, the vacuum will go up to like 17-18 inches (for like 10-30 seconds), then the egine will start to sort of cycle down the rpms and eventually the vaccum drops below 10 and the engine dies. After I restart it and drive for a little while, the idle starts surging and the vacuum jumps up and down between like 10-12 inches (I notice this at a stop light for example). Once the truck is fully warmed up, I can get my vacuum to hang at like 15 inches (real constant -- no flutuations). Another problem that I have that I don't know whether is related or not is that sometimes when I am driving at like 55-65 and I let off the gas, I can feel the engine sort of bog as I decelerate, like I dropped a parachute out the back of the ride.

I am not throwing any codes. I don't have my TAD or TAB hooked up, so I get those codes in an engine on test.

I am trying to time the beast now, but my dizzy has decided that it won't rotate (it's corroded in place, so I am trying to lube it out with PB Bblaster).

I am suspicous of a vacuum leak, but I am under the impression that a leak above the intake would be compensated for by a high idle. Could the seal under the intake be leaky, or would the engine compensate for that as well? I am not sure where the O2 sensor is in the air/fuel stream (if that's what tells my engine to increase/decrease throttle to compensate lean/rich). Can I just snug up the intake bolts a little, or would I risk squihing out the gasket even more?

Thanks for any help,

deedge

 

I also forgot to say that she's running a little hotter than she used to, but only when I am going above like 55-65mph. The beast warms up quickly (to like the A in normal on the gauge), but cools back down to R pretty quickly.

 

Yeah.. that thing has issues. On a truck that old you want to check for rusted vacuum resovoirs(juice can), these can hide a vacuum leak. You can also test for a vacuum or gasket leak by spraying the areas in question with the gas from an unlit propane torch, the motor should rev a little if the gas gets pulled in.

If you have the air injection system disabled you should plug the heads and exhaust where the hard lines are connected.. if you have not done this already. If the motor is allowed to pull in air upstream of the O2 sensor the A/f ratio will get skewed. The O2 sensor will be either in the exhaust Y pipe near the motor or in the back of the passenger side exhaust manifold.. probably only visible from underneath the truck.

 

conaski,

The old metal vacuum reservoir was changed out a couple of years ago for a nice plastic one. I have actually never seen a reservoir that nice in my life.

I was under the impression that on my rig, the computer won't let the system run lean enough to spray flammables and get the engine surge. I tried with starting fluid around all the vacuum connections I could find. But, even spraying starting fluid into the air cleaner didn't cause the thing to surge.

I am still tring to figure out the TAB/TAD system. Right now I think all the air lines are connected, but I can't figure out how to connect up the vacuum lines to the solenoids.

 

I was reading this thread similar to a problem i have, Ive been kinda running into a brick wall with this surging on deceleration, Truck runs well while driving and idles fairly well, i Replaced IAC, TPS, and Map, none of those are the problem, Ive cleaned the EGR it looked pretty good to start with, Didn't find any cracked hoses Odometer reads 86,000 Truck sat for a long time took a look of work to get it running, One thing i found odd when i replaced the the IAC, Motorcraft calls for vented IAC and one i had was non vented, Wiring looks to factory and the IAC was (OEM Ford),I put the same NEW IAC back on the truck, Looking for maybe someone who has had this problem and was lucky enough to figure it out................1991 F-250 5.8L

 

check for codes at operating temp, see what you come up with, have a read through the TSB below.
TSB 95-15-11 NO START, HESITATION-STUMBLE- INTERMITTENT STALL 4.9L, 5.0l, 5.8l AND 7.5l ENGINES.pdf

 

Thank you for replying to my post, I couldn't read the PDF attachment wouldn't open?

 

Try this one: https://www.ford-trucks.com/forums/a...1&d=1450922816

 

Thanks for the link much appreciated