97 ford f250 w/460 gas- missing slightly at low speed rpms
 

I have a couple of questions:
First- My 97 f-250 heavyduty with a 460 gas engine has a slight miss while running down the road. When I put my foot into it, it runs fine. I first noticed it when I ran a tank very low on fuel and it missed a couple times at the higher speed and I switched tanks right away, but it still does it now at the lower speeds when I'm in a high gear, and it does it on either tank. Could it be fuel filter or pump(s) or maybe something unrelated?

Second- There is quite a build up of oil in the air filter box, so I thought to myself, this is an easy one, it's a faulty PCV valve. Well, that's all fine and good, but where the heck is the PCV on this engine?! I can't find it. The vent hose from the airbox to the valve cover attaches to the oil fill spout, but it looks open with no PCV, and the parts that I've found online don't match any pictures of anything I can find on my truck.

Any help would be greatly appreciated.
Thanks!

Unless someone comes up with an exact location of the PCV valve why not try this.
Look for any large (3/8) hose coming off the base of the carb or intake manifold and follow it . One of them, if more than one, should go to the PCV valve.

If memory serves correctly, the PCV came off the front center of the engine, right above the bottom of the intake, and ran over to the Right Rear Valve cover to the valve. The adaptors and spacers were always leaking, and I think a can of carb cleaner and a good light would help finding where something is leaking. It should be a easy matter to fix. And don't be afaid to steam clean these engines. I've found that can save time. Just make sure you don't get water into the engine, and you should be fine.

Thanks for the replies...
There's is nothing (hose connections of any kind) on either valve cover that I can find, except where the oil fill spout is, and that line goes straight to the air-box filter (where the oil is leaking into the air-box). The location described sounds logical, but I have traced only line on valve cover, so that's why I'm perplexed. I'll take another look to make sure I didn't miss something.

Anyone have any ideas on the "missing" engine? Thanks again for the replies.

Yes, unfortnately, I do have an idea about the airbox. If oil is getting into the airbox, it's usually a sign of poor sealing in the engine (rings), that gets pushed into the crankcase as blowby, then deposits in the airbox, where it condenses. That happens in high milage, usually well used cars. However, the PCV is absoluetly a direct cause, so DO check it out first.

The engine has to have a PCV valve. And since they don't want any additional oil vapors contaminating any particular cylinder they feed the PCV line into the area just behind the throttle body so the vapors are evenly distributed to all the cyls. . I previously made the mistake of thinking a 460 was a carb engine. Trace the larger vacuum hoses from the throttle body or upper intake manifold and it should lead you to the PCV valve.
What kind of maintenance has been done to this engine like plugs and filters?
Did you check all the vacuum hoses for a split or broken hose?

Ok! That one was an I-d-10-t error! IDIOT. I found the pcv, and it was on the valve cover, no good excuses except to say I didn't lift up one of the hoses running across it as it was hidden underneath. I was so sure there wasn't one on the valve cover that I thought one of you guys came over in the night and tapped one in, just to prove yourself right! Anyway sorry about that. Here's the deal on the missing engine. Fuel consumption is terrible, and the missing is getting a little worse. I can't get 8mpg, and my buddy was following me this weekend while we were hunting and said it smelled super rich. Could I have a dead cylinder? Misfire? I have also noticed what seems to be an exhaust leak. Here's the full vehicle history as I know it. I bought it in fall of 08 for $2k with 125k. It was auctioned off as a returned fleet vehicle that had it's whole life offroad checking wells. The guy I bought it from said it had a miss in it, so he replaced distributor and plugs/wires, and they did look new. He said he also replace a map sensor, but I'm not sure if that's true or not. Anyway, it didn't miss at all for a couple of years. I dont' drive it much, maybe 8k/yr. Anyway, last fall during hunting season it was missing and it ended up being the catalytic converter, so we cut it out and straight piped it. And it still had a bit of a miss at low rpms, and had a higher sounding exhaust, to be expected. Then, this fall I noticed it missing more often, so that's why I am asking about if a plug could go bad so soon, and if that could be my problem or if maybe it's something else. Thanks for all the help so far and thanks in advance for any more help you can offer.
--eric

An exhaust leak can throw off the computer and cause a rich condition which might explain the poor mileage rich smell from the exhaust etc.
Spark plugs do go bad for many reasons some of which could be poor engine sealing, careless installation, misfire in a particular cylinder (spark plug wire)etc.
I would suggest getting the exhaust leak corrected and find the reason for the miss and correct it. The 460's weren't exactly great for high MPG. Let us know what you find. Thanks

From some recent and painful adventures with misfires and hesitations on my own car, I learned that an intake air leak - located downstream from the MAF sensor, which means going undetected by the engine controller - can cause a rich mixture. This is counter-intuitive, but here's what I've been explained, and it makes a lot of sense: the extra air will initially add to the metered air, so the fuel mix will be initially lean. However, the O2 sensors will see too much oxygen in the exhaust gases, so the PCM will increase fuel dispensing to make the O2 content in the exhaust come back where it's supposed to be. That way, at any given RPM, your engine will spray in a lot more gas in the hope to have it burned with all the air that's apparently getting through. However, this is hardly smooth run for the engine, as the parasitic air can't be controlled ... and its effects are more obvious at lower RPMs, when the vacuum is higher and the throttle lets little air go through.

One dead cylinder would not explain the poor fuel economy, even if it explains the smell of gas in the exhaust. A rich mix, on the other hand, explains both the poor mileage and the smell of gas. Get some carb cleaner and spray onto the intake manifold seals, as well as all other parts and joints in the air intake system. The engine should change its pace once you hit the hole...

You should also be checking the oxygen sensors, all of them. A dead O2 sensor will signal the PCM as if there was a lot of oxygen in the exhaust, which leads to richer mix to compensate. Speaking of which: is your car running as bad when you first start it up in the day (cold engine)? If it's not, then your O2 sensors are to blame (because the PCM ignores them during open-loop operation, which happens for a minute or two after a cold start). If it is running rough, then your O2 sensors are still alive ...


My car was also running rough, but my fuel consumption went the other way (a lot better than I was used to see). It turned out that I had some bad spark plug cables (to explain the roughness) ... however, to this day, I have no idea why my car was getting 30% better fuel mileage during episodes of missing and stutter.

HTH ... good luck!

Ok, guys; just think of one thing. Maybe, just maybe we'd better check to see if the compresssion on the engine (which has already deposited oil in the airbox) is OK to even ignite and burn the mixture in the first place? You've hammered in the idea that this truck was accquired with a misfire. I'm sure it does run rough; you've mentioned that throughout. And it's getting worse. Just make sure that if you do a compression test, and make sure the engine CAN run well.
If you need to yank the engine to repair the head/block problem you have, the exhaust manifold that's probably cracked in half won't be such a bad repair. I know it seems an evil thing to say, but it hurts less now when you know what you're getting into then later.
Take a good long hard look at the plugs. Any oil on any of them (and I'll bet clyinders 4,7 and 8 are dark) is bad. That can be done anytime, and without much effort.
After you diag the misfire, you can prioitize whatever you have to repair rationally.

mark