1990 F150 5.0 - very rough idle

psalmthirteen · #1 07-02-2011, 12:07 AM

Hi all,

I have a 1990 F150 5.0L that had been sitting for 3 years and I finally got a chance to take it to a mechanic this week. Long story short, it didn't run when I brought it to the mechanic (due to the distributer being 180 off), they got it started up, but then they found it's running rough. It'll shut off occasionally while idling, but smooths out when you get the RPM's up.

Took the truck out of the shop once they got it started and fixed a couple other things and brought it home due to money issues. Back before it had sat for 3 years, it didn't have this missing/rough idling issue, it ran quite well.

I'd greatly appreciate any help troubleshooting this problem. So far from what I've researched, It looks like I'm going to check the IAC, TPS, EGR, and fuel regulator.

Thank you

glc · #2 07-02-2011, 01:21 PM

Drain the fuel tanks, change the fuel filter, put some fresh gas and Techron in it, and consider changing the plugs, wires, rotor, and distributor cap. Make sure the timing is set correctly.

Labnerd · #3 07-02-2011, 01:53 PM

Considering that the engine sat for the years, the probabilities are high that you have some stuck lifters/valves or that they are not fully functioning due to sludge, coking, etc. You'll need to flush the oil system. I'd avoid any solvent based cleaners like Marvel Mystery oil (there's no mystery- it's all solvents). I'd suggest locating some of the new G Oil that you probably can find at Walmart. They have a coupon on their website for $10.00 off of a 6 pack. The G Oil is nothing like anything you've ever used before. Because of the nature of the base oil which is ester, it has tremendous solvency naturally yet provides incredible lubrication. I'd suggest changing the oil using their 5w-30 and give it a run. Change back to your favorite brand after 2500 miles. You'll probably see the idle clear up in the first 50 miles. I'd also suggest watching the oil level while using G Oil. It seems it works so well in cleaning/solvency that it can get past seals and worn rings. If you get a french fry smell from the exhaust, watch it carefully-LOL.

psalmthirteen · #4 07-02-2011, 03:49 PM

Thank you for the replies! Just to give an update on all of it.. The mechanic had changed the spark plugs, tightened the rocker arms, and checked the belts. Then once I got it out, I put fresh fuel in it along with a can of sea foam in the gas tank. My dad and I didn't flush the fuel system, although it would have been better if we did.

That was all yesterday... Today, we checked the IAC and cleaned it. Our code reader gave quite a few errors on the TPS before cleaning the IAC, but after the IAC was cleaned, the errors went down and the roughness also got better. It can idle now without shutting off and it's a tad bit smoother.

I just went up to AutoZone and bought a Duralast IAC, so we're going to try replacing that. Then we have all new wires, and a new fuel filter to put in. I'm also wanting to drop some sea foam into the vacuum line.

We had just changed the oil maybe 2 months ago, while it was sitting and not running, with royal purple. So my dad will likely be quite hesitant to flush the oil system again lol.

So with all that said, I'll update again to see if the new IAC, new wires and sea foam through vacuum line does anything for us.

psalmthirteen · #5 07-02-2011, 10:22 PM

Update #2. Replaced the IAC with a new one, now it won't idle, it'll go up to about 1000-1400 rpm or so after starting, then immediately shut off. You pretty much have to floor it the second you turn it on to keep it running. I suppose this may not be a bad thing, seeing as the old IAC was so clogged with carbon it may have just been holding it up in a crazy idle instead of letting it shut down.

Code reader still says TPS this and TPS that, so we're taking the throttle body off tomorrow to clean it thoroughly and replace the TPS.

Labnerd, I still want to try that oil out though! I'll try and pick some up tomorrow at Walmart.

booba5185 · #6 07-03-2011, 03:45 PM

Before you go through all that to pull the TPS, go test it.
http://fordfuelinjection.com/?p=30

psalmthirteen · #7 07-03-2011, 04:23 PM

Booba5185,

We ended up changing the TPS earlier today... Along with thoroughly cleaning the throttle body. Put it all back together, and it is still missing.. It'll idle up to 1400 then drop to 600 and sit there a little bit, but it's sketchy. Sometimes it'll shut off, sometimes it'll stay idling. Nonetheless, the problem of missing and running rough is still there. (New TPS did make the code reader stop displaying TPS issues though)

The EGR seemed to be fine as well... Running out of ideas... :S

glc · #8 07-03-2011, 04:57 PM

Duralast IAC? I'll give you about 50-50 odds it's defective. However, I doubt that this is the only issue.

You also may have issues if you use Duralast plug wires.

Has your mechanic done a compression and a cylinder balance test yet? Have you tested the EGR system yet? There should be NO vacuum on the EGR valve at idle, and you should be able to pull the vacuum line off the valve, connect another vacuum line to it, and suck on it - and the engine should stumble and possibly stall. The EGR valve might be partially stuck open, that's gonna make it idle and run like crap.

psalmthirteen · #9 07-03-2011, 05:12 PM

The new Duralast IAC is essentially doing what the cleaned up old IAC did, kept it idling without shutting off.

Don't have Duralast plug wires, the old ones are still on there. The new ones we have are presto- something, haven't put them on yet.

Well, one of the first things the mechanic said when it was in their shop is that there was no compression in one cylinder, and that it ran rough. Supposedly they fixed the compression issue, but the mechanics were a bunch of incompetent pocket-pickers, so not much confidence in the little that they did.

Hmm, I'll double check the EGR. When we had it off, we pushed up and down on it and it did seem to work okay.

glc · #10 07-03-2011, 05:54 PM

Do another compression test.

psalmthirteen · #11 07-03-2011, 09:15 PM

Checked all the new spark plugs that were installed last week, they all look good except for one of them. Changed the wire for that dirty/bad plug. Idling was slightly better. Compression was fine on all of them as far as I know......
My dad thinks possibly a burnt valve? Since only one spark plug was messed up, others looked totally fine, and they are brand new, only being in the truck for 4 days.

glc · #12 07-03-2011, 10:29 PM

A burnt valve would show up on a compression and cylinder balance/leakdown test.

psalmthirteen · #13 07-03-2011, 10:54 PM

Yeah, after discussing it a little bit more, we're leaning more towards the distributor cap.. Since only one plug was showing wear. Suppose we'll change the dist cap, if that doesn't work out, what would the next step be? Thanks for the help, I do appreciate it.

Labnerd · #14 07-03-2011, 11:27 PM

You need to put a vacuum gauge on it before going any farther. If it bounces around, you have a valve that's not sealing correctly. The issue can be it's burnt, or like I said in the above post, from it sitting, you have a valve or lifter not fully functioning because of sludge, varnish, gum in the system. If it sat 3 years, you HAVE this problem and until you deal with it, all you'll do is throw parts at it. If you replace the dizzy cap, make sure the cap has the copper/brass contacts and not aluminum. If you're buying one with aluminum contacts, save yer money for a decent cap. The aluminum won't last 10,000 miles and will start to go downhill with the first turn of the engine.

psalmthirteen · #15 07-03-2011, 11:41 PM

I would have checked it with a vacuum gauge as well as changing the oil with what you mentioned, but since I'm not doing all of this by myself, I have resistance from my father to do certain things on it.

Say I change the oil with that new G stuff, you said it would help within the first 50 miles or so, but I can't drive it anywhere.. So, I'll change the oil and run it for a while? No oil additive needed at all?

Also with the dist cap, we'd just be trying out the old cap for our old distributor, rather than buying a new one yet.