5.8L (truck) -- plug gap and MPG

[monterey1962]

This is regarding the 5.8L EFI351W in my '95 F-150, but might be a good general 302/351 issue.

I changed spark plugs in the truck at 90K miles, recently. Removed Motorcraft, replaced with Motorcraft. Don't know how long the old plugs were in there, but they didn't look too rough. However, all of them were gapped at .35, whereas the standard for this engine is .42-.46. I gapped all of the new spark plugs at .44.

After doing one fill up of fuel since, I've found that I've lost 3 mpg. 10 mpg dropped to 7 mpg. All in town driving with front hubs locked most of the time. Only other things changed on the truck around the time of the spark plugs were as follows: Bosch O2 sensor, fuel filter, exhaust (removed 2nd converter and installed pipe with turbo muffler), distributor cap and rotor, 9mm plug wires, battery, oil change, belt.

Even with the growly turbo muffler, I don't think that I was driving the truck any more aggressively since the change of parts. I baby this truck like you wouldn't believe. The truck idles and runs smoothly, much better than before. The weather has been nasty cold and the truck does get a fair amount of idle time in, but no more than before.

I'm open to all relevant suggestions on this. Thanks.

[mrltd]

The wider gap probably cant make a good spark. Maybe the coil or the cap & rotor, since the wires are recent

Also, the hubs being locked hurt milage pretty bad, that drags the front axle and drivetrain.

[monterey1962]

Hubs were locked during the use of previous tanks of fuel, as well.

Would getting one of those inline spark testers possibly be worth the money? I'm reusing the set of 9mm wires that were on S.U. last year. I had changed plugs and wires on S.U. due to some running issues at the track, but that turned out to be the Autolite plugs.

Anything to look for on the coil? The post on the coil didn't have any corrosion on it when I changed wires.

[Southern_Pride]

Something is definitely off. Try closing the gap back up and see if it helps. If it does it may need a coil. Mileage figures sound off anyway. Only time I got 7mpg in mine was with my 38" super swampers. And my foot was made out of lead.

[monterey1962]

Thanks for the info, guys. I'll pick up a new coil this week. It will be easier than pulling all the plugs again, anyway. I'd really like to know why the old plugs were at .35. Each one was set perfect. G-pa didn't keep any records on the truck when he had it, so I'm probably never gonna know the answer.

I'm still open to input on this and I'll keep you guys up to date as to what I find.

[85crownHPP]

how often do you check your tire pressure?

[gadget73]

Could be an old timer installed the plugs. Smaller gaps were used on points ignitions because they weren't as hot. Could also be someone closed them up to fix a running issue that was caused by some other problem.

[Southern_Pride]

I don't blame for not wanting to pull the plugs again. Passenger side sucks big time on those. I wish I remember what mine were set at when I threw em but I just threw em in and didn't bother to check em. I was kind of in a hurry.

[monterey1962]

how often do you check your tire pressure?

It's been a couple months. Two new tires went on in November.

Could be an old timer installed the plugs. Smaller gaps were used on points ignitions because they weren't as hot. Could also be someone closed them up to fix a running issue that was caused by some other problem.

Actually, the "old timer" part could be right. G-pa farmed near a small town community and my uncle mentioned in an e-mail that the truck went in for some work regarding the radiator, once. Maybe plugs were done, too. That would explain why they didn't look as old as I was expecting.

Also, I'll pull codes first chance I get, see if anything shows up.

[monterey1962]

Passenger side sucks big time on those.

I don't get it. My uncle said the same thing about his 5.8L. When I did mine, I had very little issue accessing the passenger side plugs. All original under-hood equipment seems to be in place, too.

[Southern_Pride]

Maybe yours is oddball. Mine almost touched the inner fender on that side.

[monterey1962]

Installed new coil tonight. I swear it idles a bit smoother and is a little more responsive on the throttle, but it could be in my head. So ... tired. Will see how it acts over the next few days.

[Rob Roy]

If you still aren't happy with the trucks performance after replacing the stock ignition components, consider adding a capacitive discharge ignition box. MSD 6A boxes are all over ebay, used ones are often less than $100. I run a Mallory Hyfire on my camaro, it really improved the idle quality and throttle response.

[monterey1962]

Alright, so it was probably in my head. Pulled codes tonight:

KOEO: Code 332 - Insufficient EGR flow detected

KOER: Code 538 - Insufficient RPM change during Dynamic Response Test -or- Invalid cylinder balance test (throttle position movement) -or- Invalid cylinder balance test (cylinder identification problems).

Anyone care to toss me some advice on this? I tested a different plug wire on #6 (just to make sure the wire was good) and there was no change. I don't quite understand the Code 538. All info is appreciated.

[gadget73]

Did you step on the throttle when it was testing itself? If not, theres your 538. Could also be you didn't give it enough throttle.