RPM Limit Strategies
 

I've been chasing a roughness/miss/stumble which has remained through SD to MAF & into the change to a HOG0 computer.

I've got good vacuum 17-18 inches at idle, 36 lbs fuel pressure, new wires & copper plugs. Cap & rotor new.

I was goosing the throttle this afternoon & watching the rail mounted gauge and at 5500, I got a big miss that went away immediately by dropping RPMs just under 5500.

Never hit 40 lbs on the FP gauge.

Recently when I changed the fuel filter, I rigged up a switch to the diagnostic port & a clear hose to a glass gallon jug & tested the pumps out put.

I can't remember the actual time to fill, but both pumps put out, (by my calculations at the time [I think] ) 90 gallons per hour which I think is about right.

Can someone tell me when the ECU stratagem is to drop injectors to limit engine speed as well as what pump output should be?

Suggestions?

I haven't revved mine out since she crossed the 200k mark. But before then I did it a few times, and as accurate as the dash tachometer is, it seemed to be around 52-5400 on my SD 302 95 truck with a 0joy pcm. Hope that helps some what.

Thanks for that tidbit. With lack of info, anything is better than nothing.

Can't find my notes, so I may run the fuel test again.

Trying to find the pumps ratings, but nothing shows up but stock 90's Fox stuff.

Straight from the Ford Service manual:

Thanks Randy, I knew that had been posted before but no matter how I parsed it, externally or internally to FTE I couldn't pull it up.

If it's on my service disc, shame on me.

I think what I measured (unrestricted free flow) was 1.19 sec/gal rear & 1.14 front or roughly 72 & 68 gph each.

I thought I had read here before that the EEC dropped injector grounding on alternate banks after a certain RPM, but have no attribution for such.

I was trying to see if I was getting a full 40 lbs on the rail when it hit the 5500 mark & the started cutting out. It also seemed to start doing that easier (lower RPM) as I tried again.

I don't think I ever saw 38-39 lbs on the gauge, so I guess I'm going to have to do something as to be able to drive & monitor FP

No worries David, the fuel supply info seems to be buried on the service CD. I recall from your dyno pull video it seems like the computer was pulling fuel. But the only way to tell for sure is verifying fuel supply under load. Good luck, be careful :-jammin

Something else to consider is a weak coil. If it cannot fully saturate before firing. It can cause a stumble because of lack of a complete burn then the O2 sees it as rich and pulls fuel when it's a spark issue not a fuel issue.
If it's not going rich and assuming it needs to lean out it could also not be firing correctly
Had personal experience chasing a fuel issue in a friends 95 5.0 4r70w truck. I'd could give a list of mods but let's just say the 5.0 was better suited for a mustang. Anyways, in high range it would run good up to 45-4700 then nosedive and backfire like it was going lean. So we chased fuel and checked for bad injectors. Nothing at all. He got all fired up and put it in low to take out some frustration and played into it. At 4000 the problem started, well fuel requirements are the same for a give rpm so what gives. Swapped the coil with a different one and no more problems. All we could figure was it was good enough to drive on, but to weak to fully charge at elevated rpms and the faster revs allowed by low range exacerbated the problem by giving less time to charge.

Hey it's an idea. It has a Mallory (slightly hotter than stock, no longer made) that I've had on it for 12 years that always has kept the plugs clean.

I'll dig out the original Motorcraft.

His was a crane fireball. Went back to a motorcraft and was all better

I suggest a wideband and fuel pressure gauge for your truck. It's not a cheap combo, but it's pretty much crucial to have when you start to mod just about any gasser. It'll help you with immediate diagnostics, and long term monitoring/familiarization with your setup.

Since you have the issue through changes in management, I'd be looking at fuel pressure (which you are) and injector testing, as well as ignition system.

This spring I had the manifold off & I tested the injectors at pressure & the patterns are good.

A wideband is definitely on the wish list/check list.

Dug out the coil that was on the truck & will swap it out, may or may not be original (# only, no ford logo).

Has a big chunk out of the output terminal.

Step at a time.

Dug out the original clip style (150K miles) coil wire & plugged up the wires to the coil missing a chunk of aluminum in its terminal...

And got an improvement. Can't say it's 100%, but noticeably much better.

Have MC coil on order, will update later.

My OEM MC coil is 26 years old and has never let me down. It has reached 5500 RPM several times and that's when my cam gives out, but never stumbles.

I keep spare MC coils and ICM's in all of my trucks for peace of mind.

I've got all manner of tested used spares save a MC coil.

Right after I got the truck, I started getting the die & wait, restart, with progressively longer times "problem".

After changing the ICM, then coil, I finally replaced the pip with a MC unit even though I had no pip codes & it took care of it.

When I rebuilt the engine I checked the play in the distributor shaft & it was acceptable

The coil in the truck when I got it, I have my doubts about, but it is better now.

Can't transfer pics of it off my phone for some reason right now.

I tried to brush off some of the white corrosion with my bench grinder's wire wheel & another chunk blew off even barely touching it, so I stopped.

Still concerned I never pulled 40 lbs before I hit 5500.

You'll never see 40+ psi from a free rev, the throttle blades aren't open enough to drop vac to the point of demanding max fuel pressure from the fpr.

A load test is the only way to confirm that your fuel pressure doesn't increase with vacuum drop. Unless, of course, your pump is so weak that it shows up as a decrease in pressure with a free rev.