Ok here it goes (a long one but please stay with me!)

I did a roller swap into my 1988 5.0 F250. I bought a complete long block roller setup with gt40P heads. The cam that they installed was a B-Cam (I wanted the E-Cam, Ohh Well). I installed the engine and used all of my accessories. I left the knock sensor disconnected because there wasn't a port drilled in my new engine.

I've been troubleshooting this for about 2 months and It's getting closer but still not quite right. The rig idle's ok but a little rough (like a large cam) the problem is that it feels like it is missing and sputtering until I reach about 3000 RPM's then it runs like a raped ape. It feel's like I have about two cylinders not firing until I hit 3000.

I've checked wires, plugs, gaps, compression, computer codes, vacuum leaks, and connections to no avail. The only thing that I had a problem with was vacuum. I have 11 pounds at idle. This seems low but I was told with the B-cam to expect low vacuum. I've tried both systems (mass air and Speed density) and the mass air system definatley runs better but not right. I've removed the intakes and had then plained to no avail. No change in vacuum. I've even hooked a smoke machine up to it and found no leaks.

I'm thinking that maybe with the low vacuum that something isn't running right (remember with the mass air there is no Map sensor used for vacuum). I'm thinking that maybe I should install a different cam. What do you all think?

 

Sorry, can't help you with your problem. Just
wanted to let you know that's one good looking
truck. Which rims do you have on there ? They
look like M/T baja's ! Good luck with your
project. Dan

 

What about firing order? Does this motor have a different firing order than your old non-roller motor?

What is the duration of the cam? Is a B-cam have have a significantly different valve timing than a E-cam?

Just curious about the motor, I've heard that mass air 302's require more fuel pressure than speed density engines. Any idea on this?

 

Thanks about the truck, and they are American racing Baja 9.75x16.5.

I've changed the firing order to the 351 order. I've never heard about the fuel pressure! I would think if that was the case it would run worse when I put my foot into it. But it runs fine at high rpm's.

 

Maybe you should just keep it in 1st and second gear around town to stay in the upper range. No, seriously im sorry to hear all the trouble your having. Did you happen to change to a new computer? Such as the mustang A9L? I had thought you were talking about that earlier, can't remember. What about the injector order? There was a debate a while ago whether or not that was needed to be changed along with the firing order. But I think that didn't matter, at least if your running speed density batch fire. How old is your ignition coil? Seems kind of wierd it would run rough then clear up higher in the powerband. What about Ignition timing? I don't really know what to tell you...

Doesn't the mass air system use something called a BAP sensor? For some reason I thought it was related to the MAP sensor on a speed density system.

I hope you figure this out soon. I know how frustrating it can be!!

As a side note, how is the power of your engine? I suppose you couldn't comment on low end since that is were your having the trouble but how about upper end?

 

I swapped the injector order around, The mass air swap came with a mustang computer, and the coil and distributer are brand new (I tried the old ones too!). The mass air system changes the MAP sensor to a BAP sensor. And the timing has been changed back and forth with no change.

The wierd thing is if I give it just a little bit of fuel it will run good on low end. But if I give it more it will sputter and miss.

The power is unbelieveable up top. It will put you back in the seat from 3300 to 5000. It's just that from 800 to 3300 it hardly pulls itself. But watch out when I run through town in 1st!!!!!

 

Ok, I have thought of a few more things...

Did you change injector size? This might sound crazy but ive heard of some people having trouble with certain kinds of spark plugs. But from what you describe it sounds like your problem might be a little to extreme to be spark plugs. I wonder if a custom burned chip would do any good. Sometimes you need a chip when you change injectors or to change air/fuel ratios for certain needs. Problem is that is $300 hit or miss. HOpe you get it figured out soon.

Are you driving this truck daily now or just parking it till you get the problem figured out?

 

It is parked but I bought another truck so it isn't a big deal leaving it. I have thought about the chip idea and that sounds like the best bet yet. Either that or the cam swap idea. I think if I can get the vacuum up with a different cam it may run better. As far as injectors are concerned I would think that the rig would have the opposite condition than what I have right? Run great on low but bog out on high (I would think that would be the case with too small injectors) what do you think?

 

Sorry to hear of your woes, especially with as much money as you have spent. It sounds like a real problem, as opposed to a tuning issue, so I would hold on burning a chip. Maybe the guy that suggested a computer swap problem was onto something, maybe you can borrow one from a mustang or something. Manual and Automatic are different, so watch out. Maybe look at the knock sensor- can you somehow use it in a jury-rigged way? It just needs to be solidly mounted somehow. Also a long shot is the plug type. It made no sense, but a friend's suburban ran horribly with platinum plugs (maybe a bad batch?).
Good luck...

 

I reread your post and for some reason the vaccum stuck out. 11 lbs seems a bit low.... I was thinking like 15 was the borderline but im not at all sure on that. Did the mustangs come with the B or the E cam? If they came with the B cam it should produce enough vaccume to operate properly since ford used it in production. That would be a bummer if you have to change the cam, I hear its alot of work.


On the injectors...This 302 was the 270HP version right? If you have the stock 19 pounders that is sufficance but they will be running at a higher duty rate. Here is a good forumla to figure injector size.

Engine HP= injector flow X number of injectors X Duty rate/BSFC

The practical injector duty is 80% although almost all vehicles come less than this.

BSFC is the Brake Specific Fuel Consumption. It is determined from engine dyno reports. Most stock vehicles run around .5, more efficient engines are a little lower.

19 (injector size) X 8 (number of injectors) X .8 (% duty)/.5(BSFC)=243.2 HP

You most likely have more HP than this but figure in a 90% duty rate and its up towards 280HP and your engine is a little more efficient also. It doesn't sound like your injectors are the problem unless you've upgraded and not told the computer what you've done.

 

I'm not telling you to go buy a chip, but I have a feeling that might be the problem. Whenever you change something fairly drastic, you have to get a chip for the MAF too. Things like injector size and air mods need the chip so the computer know's whats going on. So if i were you, poke around with the other suggestions and if we can't get anywhere, it's possible you need a chip, I know the mustang guys do.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

That's it for mods now, saving for a Stang. Check out my Gallery for a look-see.

 

Even with a B cam you should have more than 11" at idle.Look for a vacuum leak,especially where the intake meets the heads.

Billy

 

I've had the intakes off about 5 times and resealed them with new gaskets and a compound called "the Right Stuff" (a guy from ford recommended it). I even plained the intakes and again resealed it and still had 11 lbs. The only thing left was the vacuum tree and I disconnected that and installed the meter in its place and still got 11 lbs. (the PCV valve hosing is new).

Maybe the injector idea is right but I would think that they would cause the problems at high RPM rather than low RPM. I am running the stock 19#ers and the engine that is in there is rated at 330 HP.

I wish I just knew exactly what was wrong with it so I would quit throwing money at it. I hate troubleshooting like this!!!!!!!!!!!


Ohh and thanks for all of your help!

 

That B-cam is m'kay...224/224 w/ .480/.480 lift. The B-cam also starts pulling hard at about 3500rpm if I remember right. With 1.7RR's it is a bit better cam than the E-cam. That E-cam is 220/220 w/ .498/.498 lift. I hated the E-cam in my last Ford powered ride, idle problems galore! I'd get a split duration like 212/218 or 214/220. Good low an mid range power for a truck. Split duration I like a bit better than the single pattern FMS cams, also good for a power adder...

Your fuel regulator may be no-bueno or have you checked your filter...my Pops' '55 Chevy had the filter in backwards and pinched down on that car! It would just barely run, and hey it is $8 to fix. Check the idle fuel pressure too. You may or may not need more base fuel pressure with that cam/heads.

That 5.0 HO and 351 firing order is 1-3-7-2-6-5-4-8 counter clockwise. And def keep that MAF on there, SD will barely idle with a cam. That MAP sensor becomes a BAP sensor with MAF conversion, simply unplug that vac line to it and plug it.

How did the truck run with your MAF conversion before adding this motor?

Other than that I haven't a clue what else may be bad.

 

If if runs like a non-consensual primate after 3000 RPM, then the fuel is not too low pressure, and the injectors not too small. Still likely to be some electronic component mis-match, but that low idle vacuum is suspicious. Possibly the wrong cam (something wilder than a B303)?
Let us know when you figure this one out...