I decided to give my recently purchased 91 f-150 5.8 a tune up. Things really didnt look that bad, but i figured I would give it a fresh start.

I cut off the old rusty muffler and put a new one on.
Cap/rotor
oil change with 5w30 max life


When I started it up it was running fine, but seemed like maybe there was an exhaust leak from the right side.. y-pipe perhaps. I ran it for a bit and the tick went away. A friend said his 302 f150 does that occasionally when he changes the oil..

I put about 20 miles on it and the noise kept coming and going, but otherwise performance was fine...

Yesterday I go to drive it to work and the noise is there, I get on the interstate and it has no *****... I get off the interstate and it is running rough as hell, oil pressure is dropping below the low mark at idle, but with any rpms at all it shoots back up. It just seems like it is missing on a few cylinders...

I throw some lucas oil in thinking maybe ut really just hates the 5w30 with no change.

Put new cap and then original cap with no change.

I threw a timing light on every plug wire and they are all getting juice at what to the eye seems to be a regular pace..

What should I be looking at?? Im frikking lost. To go from running fine to **** like this has me totally lost. Im pulling plugs right now and checking compression, but is there anything else I should be doing?

I checked the Check engine light and got this

539-- ac on during test
539-- ac on during test
1- ??
332- egr not opening
637- trans temp switch failure
332- egr
637 trans

Thanks for any help. I need to get this truck back on the road!

 

Well how is the oil level/condition? Do you have a mechanical gauge you can put on it as a test? It sounds like you lost oil pressure at one point.

Also beware the factory oil gauge ain't set up to operate like you think it does. There is a pressure switch that closes above ~7 PSI oil pressure. This will drive the gauge's signal circuit low (shorts it to ground) thru a resistor which will cause the gauge pointer to move "somewhere" within the Normal band. When oil pressure drops below 7 PSI or so then the switch opens and the gauge takes a dive into the red. You can jump the resistor and install an actual pressure sensor to make it work like an oil gauge should.

 

oil is new and slightly above full....Im thinking the idle probably dropped below normal do to it running so ****ty...causing the oil pressure to drop..

 

Your oil pressure gauge might be doing funny things but I doubt the problem is oil related. I would focus on the cap and rotor.

 

I would go back and check to make sure you got the firing order right.

 

I copied what was existing, but just for kicks, does anyone have a diagram?

 

You said you did a muffler cap/rotor and oil did you do plugs
A sticky egr can cause it to run like doo doo

 

I checked the egr and found a vacuum line bad.. fixed that and still no real change though I can tell you a little more about it..

Ive noticed in testing that it will idle fine, accellerate like a turd and once up to speed any small press of the pedal will cause it to shake like hell, but if i floor it it runs smoothly...and it will cruise at almost no pedal input smoothly..

 

Did you change the plugs are they gapped correctly

 

You sure you didn't leave something unplugged? TPS or MAP sensor maybe? It don't sound like an oil issue now, more like fuel or ignition. Does it misfire while it's accelerating like a turd?

 

Another vote for firing order.

1-3-7-2-6-5-4-8



Also may be time to do a cylinder balance test to find the faulty cylinders that are not contributing.

Part 1 -How to Do a Cylinder Balance Test (Ford 5.0L, 5.8L)

 

Firing order is good, ill try that balance test

 

Well hell.

balance test worked great.. and showed me cyl 5 bad.. pulled it, plug covered in oil, and no compression.... so what is my next step? next cheapest step?

 

Well, the oil is coming from somewhere. Leave the scary stuff like cracked heads and damaged pistons alone for now. First step would be a compression test. All cylinders even? Maybe throw a new plug into #5 and see if it runs better?

Oops, missed where you said no compression. If so, time to pull the head.

Really can't see basic tune up causing this, something let loose.

 

Sounds like a case of bad timing and the engine letting loose right after a tune up.