Hi folks,

I recently bought an '89 F250 with a 351W/5Sp tranny in it. I just added a tach to it and discovered it doesn't want to rev higher than 2500 rpm. I'm sure it has an RV type cam in it, but still that seems awfully low. I'm not sure how many miles it has... it says 52K. And, it's not throwing any codes. The first thing that came to mind was a problem with the mechanical timing advance, but the engine is so loaded with tubes and hoses going everywhere that I was hoping for some input first. What do you think? Is that normal... 2500 rpm?

Jim

 

This is not normal. Are you sure your tach is reading correctly? Most aftermarket tachs can be set to work on 4, 6, and 8 cylinder engines. Make sure it is on 8.

 

Yes, it's set for 8 cylinders. Guess I better check the timing and see if it's advancing properly. I can't think of anything else it would be. I don't believe it's a fuel pressure problem, but I'll look at that too.

 

Thats got to be it, the 351 should easily tach. 3,500 rpms on a moderately hard acceleration, probably more.

 

What does the truck do when you get up to 2500rpm? What do you mean by "doesn't want to rev higher"?

 

Jim Sounds like the tack... Did you buy it new / What kind of tack??... What does it read at idle ..I dont see you say anything about the way it runs ..If it would only rev to 2500 you might have a different post.. Lew

 

Is the truck running alright? If it is running fine, the problem has to be the tach, my 351 is turning more than 2500 RPM when cruising on the highway.

 

Go out on a quiet street or parking lot,put it in 1st gear,and slowly accelerate......1st gear as i recall is fairly low,not a true "granny" low,but around 4-1/2 or 5 to 1 ratio,so i would guess that around 15 or 20 mph at most you should be turning quite a bit more than 2500 rpm,and if not it sounds like the tach is improperly set. Lew52 brought up an excellent observation regarding RPM's at idle,which i would expect to be at around 600 rpms or so,once the engine has reached operating temp.....if you are showing significantly lower rpm's than that,it would likely confirm tach isnt reading properly.....OLd93 mentions aftermarket tachs being adjustable,usually there is a little selector switch on the back of the case that must be set for 4,6, or 8 cyl. engines,make sure yours is set correctly!

 

Guys,

The truck runs strong off idle and pulls strong up to about 2000 rpm. But, in neutral it just doesn't want to go any higher than about 2500. I'm convinced it's a timing problem. Here's a little more background... when I bought it, it had a bad oil leak from the front left valve cover and had caked on oil all over the front left of the engine, including the distributor. That's the first thing I fixed. The truck idles nicely at about 800. It looks like it's turning about 2200 rpm at 70 mph. I just took the tach off another truck where it worked correctly. I don't think the tach is the issue. I put a timing light on it but it was difficult to see. The timing was moving some, but it didn't look like it was moving as far as it should. Eventually, I'm going to have to clean up the timing marks so I can read it more easily, but the first thing I'm going to do is get rid of all those air tubes, so I can get to the engine. Then, I'm going to look inside the distributor where I fully expect to see something wrong... more heavy oil deposits or maybe something broken.

It'll take me a couple of days to get this done, but I'll let everyone know how it turns out. Thanks for all the input.

Jim

 

i've never heard of that happening, could your timing chain have jumped a tooth? these cams are so small you might never know it, what does it do when it is at 2500rpms? feel like it's hitting a revlimiter or sputter or what and i assume your at wide open throttle when it's not reving right? maybe hold it in first and just stand on it and see how many rpms you can get out of it i shift my 302 at 6000 with the stock 5/16 rod bolts, 351s have 3/8ths rod bolts you should be able to get 6500 or so a few times with no damage done

 

i got my old 351 to 6700 once

was in 4low and i had the trans in 1st i forgot.. to shift it in D lol i was scared but nothing happend tough

 

Some engines have rev limiters in neutral, I wouldn't this would be one of them. When you start pulling tubes off the engine, you may open up another can of worms.

See if there is a lot of slack in the timing chain. Take the cap off the distributor. Rotate engine manually till the rotor moves. Then reverse rotation and see how many degrees of crank rotation you have before it moves. More than a little, you've got a lot of slack and enough to jump a tooth.

Sometimes a bad fuel pump or system will cause fuel starvation at higher revs with results like this.

 

I read some where in anouther post about a neutral rev limiter ?? I used finger nail polish to mark the balencer at 0 tdc... & 10deg before tdc two differant colors so they are easy to see...When you check the timming you will be able to see whats up... They usally stay close in time unless you have a wear problem....Timming chain.. dist.. so on ...Dont for get to disconect the advance before you check the timming... Good luck... Lew

 

Hey Jim,

I would not do this yet: If you start disconnecting stuff, you could cause some (new) fault codes before you even get the existing fault codes out of the ECM. I would pull the codes first, before you even consider disconnecting "all those air tubes..." to avoid adding to the confusion. In fact, given that stuff may break when you attempt to disconnect, I would be leary of doing any disconnecting at all, prior to getting a real diagnosis. Just my .02.

Also, though I'm not a big engine-internals guy, I would consider, as has been mentioned, the possibility that the cam has jumped a tooth (or more?) on the timing chain. In addition to the (static) chain-stretch test mentioned above, I remember reading about a test you can do with a timing light.

Perhaps someone can fill in the gaps in my memory on this, but I believe you get a rough idea about how much timing chain slack is present, with a timing light. I don't remember all the details, but I believe you would check the timing with the spout disconnected, and then vary the engine RPM, to see how much deviation or "walking around" there is in the timing? (I don't remember what is normal, under the mechanical advance, but as I say, hopefully someone can refresh us on this?). IOW, I believe that, (with the "spout" disconnected, which removes electronic spark advance) other than intended variation from any mechanical advance system (?), there should not be big changes in the timing when the RPM is varied, and anyt such changes would indicate a stretched timing chain....

And remember, if the cam has jumped time, then the ignition timing should be way off, too. Perhaps someone has a "rule of thumb" as to how many degrees of dizzy advance = 1 tooth of cam timing?

Also, myself and others would like to hear what happens, in neutral, when you attempt to exceed 2,500 rpm? Does it run well? Stutter--what?

Good luck.

 

It feels like it hits a rev limiter at 2500 in neutral. It doesn't sputter, it just stops going up. I didn't know about the rev limiter before.

Hopefully, I can get it back on the road today and test it again. It wasn't throwing any codes before and didn't appear to have any vacuum leaks.

I need to check the PCV... I assume it's in the back of the intake, similar to a 5.0?

Thanks for the all input, guys. I'll let you know what happens.

Jim