I have a 91 F-150 with a 300 6-cyl. Its a 4wd and has about 180,000 on it.

I having a problem with it missing. I'm just a shade-tree mechanic and can't seem to get it figured out. I really hate to dump a large sum of money in to the truck just to get it fixed.

The problem I am having is the truck is missing. It starts up fine, and seems to run smooth when it's first started. Once the (what I call) electric choke kicks off after about 20 seconds or so, and the rpms come back down, it starts to miss. When I rev the motor it misses. When I drive it, it seems to do it in all gears all the way up through the RPM's. When climbing a uphill slope it will even seem to backfire up into the intake. I did a tune up on the truck about 8000 miles prior to this happening. Since it has done this, I have replaced the fuel filter, spark plugs (again), MAP sensor and the Air change temp sensor.

I've also had it ran on a computer, I think it was just a generic computer and it didn't bring back any codes. The service engine soon light isn't on in the truck either.

The motor doesn't have any kind or tick or knocking noise to it.

Any kind of help or point in the right direction would be appreciated.

Thanks Josh

 

it's been a common theme this last week for clogged fuel injectors so I will throw that out there first but more probable would be the cap and rotor and maybe setting your timing. I doubt the timing though since you haven't messed with that yet and it ran fine before. Also you should get a new set of wires with the distributor cap and rotor... It makes more of a difference than you think...

 

Well I did replace the cap and rotor along with the wire about 8000 to 10000 miles ago when I did the complete tune up. Do you think it is possible that it could be these again?

Also is there anyway to check the injectors minus replacing them?

Thanks

 

well as long as you got decent wires, cap, rotor, and plugs I think you are safe that way. It could be possible that you are gaped to close or wide on you plugs... Did you gap them or just put them in????

Also as far as the injectors go check this forum...

https://www.ford-trucks.com/forums/9...-cleaning.html

I have some posts in there and there is a process for cleaning your injectors yourself (RJM). You can send in your injectors to be cleaned by witchhunter or another online site. I ran across this one this morning...

Fuel Injectors: Find fuel injectors at Motor Man Fuel Injection Supply

You can get rebuilt injectors for $20-$30 each and have the old ones for spares...

Also if you have a engine scope and can find which cylinder the miss is coming from you can try switching the injectors and spark plugs around... If the miss moves with one of the components when you move it then you've found your problem.

 

Yeah I gaped the spark plugs. I really don't think it is that, although I'm no mechanic, but I have replaced all that stuff within the last 8 to 10 thousand miles. I can't remember what I gaped them too, but I asked the guy at the auto parts store when I got them.

I did find remanufactured injectors on ebay for less then $100 for all 6 that come with a lifetime warranty.

I just really hate to spend the money unless that would be my problem. I reckon $100 isn't that bad of a deal though.

Is it possible it could be an O2 sensor? or a vacuum leak?

Thanks Again

 

Also, one more question. do you happen to have an article or something that would show me how to take these out?

Thanks

 

Check the plug wire routing. It could be arcing against a piece of metal. Try misting the wires after dark with a water bottle with a fine sprayer. Look for arcing. My only other suggestion would be the EGR.

 

I can't promise that the injectors are the problem. They were seemingly the problem of the week... IF, you want to do them I would suggest the RJM rebuild kit and a good can of injector cleaner. And if you have a air compressor that's all you need to clean them out yourself ($30-$35). I would also check all your vacuum hoses for breaks, dry rot, dis-connections, etc. If you are arcing through one of your plug wires (did you get good 8mm wires???) you can solve this with some cheap $3 plastic spacers that snap onto the wires. I would also lean toward a EGR problem... Hopefully we can get one of the more experianced member to comment. They might have a good idea where to start first and what is more likely...

Also if you were asking how to take out the fuel injectors... you just need to get the fuel rail unbolted and lifted from the fuel injectors and then the fuel injectors just pop out with a little persuasion. I can probably get pics to show you if you need them...

 

Thanks again for all your help.

I had a guy I know come over this weekend and hook it up to a computer. After running a few test it really didn't give back any codes other then low idle which he thinks is from the miss and knock sensor. So he started pulling plug wires to try and narrow down which cylinder it was. When #1 was pulled it didn't make any change in the motor. 2 thru 6 you could tell a different. So I narrowed down my problem to that. We also check that wire and it's for sure getting a good spark. The spark plug in it is brand new so I don't think it is that. I also gapped it to the auto parts store spec. It is an autolite spark plugs/ wires. I'm not real sure what mm the wires are.

So I'm leaning towards an injector problem. I found a set on ebay for around $80 that come with a lifetime warranty and no core charge. I'm thinking about picking them up and tryin them out.

87 88 89 90 91 92 F150 F250 F350 Fuel Injectors 4.9L L6 : eBay Motors (item 280466931188 end time Feb-28-10 00:21:31 PST)

Will I have to buy replacement gaskets for the intake when I take it off to replace the injectors?

I haven't check the wire yet to see if they are arcin. Also to check for a vacuum leak, can I just start the motor up, and spray ether around it to see if it speeds up or not?

Thanks Josh

 

getting rebuilt injectors is usually a good idea. They are cheaper and the rebuilder gives a better warrenty (keep the warrenty papers). I suggest anything with over a 30 day warrenty be scaned into a PDF file and saved on a flash drive and in the truck. Saves you from having to find them later, keep everything organized too! Over the next few years of ownership the injectors will pay you back in fuel savings so you aren't going to loose much on them. Oh and yes you will need a new PLENUM gasket for the break between the intake and plenum ($4-$8).

As far as the vacuum leak check goes... I just visually inspect all the rubber under the hood. Just bend the rubber back and fourth and and if cracks appear where you are bending it replace it! I usually go to the local hardware store where they sell it by the foot (cheapest option). Auto parts stores will charge you $5-$10 for a package of 3-8 ft of hose which is way more expensive. I honestly pull into the parking lot and then open the hood check the hoses and remove the bad ones (know what they sell before you do this though) and then take them in to match up length and diameter and you are good to go!

JOSH do the $20 in vacuum tubes first before you go buying injectors!

 

Ok man, sounds good.

So just standard rubber hose from the hardware store will work just fine?

Thanks again for all your help.

 

yeah it doesn't have to be fuel safe or anything just has to carry a positive or negative vacuum. There is usually a stand of rubber hose of all different sizes. it is way cheaper per foot to buy it by the foot and not by the package... Plus you don't have extra or not enough.

Before you do your injectors I would also do a search for EGR valve and read up on those. I'm not an expert on them but EGR stuff could be causing your problem too... Or you might try private messaging Conanski, Lew or any of the guys who have 5000+ posts... they can stear you in the right direction if the fuel injectors or vacuum hoses aren't the problem (even though I have 3 old ford trucks... I'm still learning myself...)

 

Ok I got my injectors the other day. Finally had a day to work on my truck and I got all the way down and get the old ones out. Now I can't get the new ones back in. What the tricks to it? I get some in and the other pop out and then I can't get them all lined up. Is there some kind of trick to this or anything? Any help would be appreciated ..

Thanks Josh

 

They don't really snap in place. There is nothing but an o-ring on each end keeping the injector in the manifold ports and in the fuel rail ports. You should just be able to put all the injectors in place and drop the fuel rail down on top of all the injectors. Keep working it until you get them all seated and the fuel rail is resting on top of the perches it bolts to.

Advice: Once you bolt the fuel rail down, cycle the key back & forth a few times to build up pressure in the fuel rail. Make sure there are no fuel leaks. Even the tiniest nick in one of the o-rings will lead to a fuel leak. You don't want to find out about it later when you run the engine the first time and fuel sprays everywhere. Ask me how I know about that!

 

If the motor is out of time, then that can make it backfire through the intake, and miss also.