F.I. Stumbling

[gtzero]

Hey All,

Wanted to see if anyone knows what this might be. If I'm just cruising around, my f.i. is fine for the most part. But if I'm having some fun and shifting beyond 5000 rpm, when I hit the gas again after the shift, I get a stumble/mis-fire in the engine. It's only for a beat and then all's fine. This behaviour has only been observed when shifting beyond 5000 rpm.

Any ideas?

Thanks,

Manny

[wrench459]

Could we assume its a 75 model?

[David McCollam]

It sounds like you have fuel starvation issue. Your car is a GT converted to FI, right? What is the fuel pressure? What diameter is the fuel line? Is the prescreen on the tank pickup clean?

Haven't we discussed this before?

[tekenaar]

It sounds like you have fuel starvation issue. Your car is a GT converted to FI, right? What is the fuel pressure? What diameter is the fuel line? Is the prescreen on the tank pickup clean?

Haven't we discussed this before?

I agree and would add that, if it's a conversion and NOT a '75 model, you will always have a certain "fuel-level-dependent" fuel delivery issues during cornering, accelerating and braking because only the '75 fuel tank has a mini-sump surrounding the fuel pickup to keep fuel from sloshing away from the pickup during those conditions!

[gtzero]

It sounds like you have fuel starvation issue. Your car is a GT converted to FI, right? What is the fuel pressure? What diameter is the fuel line? Is the prescreen on the tank pickup clean?

Haven't we discussed this before?

Ya, I know this is all probably related. The fuel pressure is right on the money. The fuel starvation around turns has disappeared recently. And this stumble has appeared. I've had my car on track before and I don't recall this stumble. So it's a new development. Actually, my car has a lot of "weird" issues since she came back from the body shop. I feel I need time to really root it all out. But time is the one thing I don't have these days.

Manny

[tekenaar]

Ya, I know this is all probably related. The fuel pressure is right on the money. The fuel starvation around turns has disappeared recently. And this stumble has appeared. I've had my car on track before and I don't recall this stumble. So it's a new development. Actually, my car has a lot of "weird" issues since she came back from the body shop. I feel I need time to really root it all out. But time is the one thing I don't have these days.

Manny

If it's been in the bodyshop, two things . . . first, re-establish ALL your electrical grounds! . . . second, did you remember to tell them to remove the battery ground cable before doing ANY welding?!

[gtzero]

If it's been in the bodyshop, two things . . . first, re-establish ALL your electrical grounds! . . . second, did you remember to tell them to remove the battery ground cable before doing ANY welding?!

No welding should have been necessary with the repairs needed. But who knows. Most of the weirdness have been electrical in nature. Such as the reverse lights suddenly not working and then a week or so later, start working again. And my flasher unit blew.

Thanks for the suggestion.

Manny

[wrench459]

One more point even if the fuel pressure is good.
Double check the fuel volume.

[deaner]

One more point even if the fuel pressure is good.
Double check the fuel volume.

This is a good point. Volume is more important than most would think. Also check the change in pressure. This is important. From full throttle to no throttle and then back on the throttle (as in high rpm shifting), to much of a drop in pressure is bad. Should be between 3-4 psi drop in pressure. You could be starving for fuel at this point. The pump must be able to keep up.]

[gtzero]

This is a good point. Volume is more important than most would think. Also check the change in pressure. This is important. From full throttle to no thorttle and then back on the thorttle (as in high rpm shifting) to much of a drop in pressure is bad. Shouldn't be more than 3-4 lbs. You could be starving for fuel at this point. The pump must be able to keep up.

Good points guys. What's the best way to check volume?

thanks,

Manny

[wrench459]

1/2 a pint in less then 15 seconds is a good starting point.
If you've got a heavy breather then you'll need more.
The way I check the volume is to hook up an inline gauge with a large cylinder. (quart)
Then measure.

[tekenaar]

1/2 a pint in less then 15 seconds is a good starting point.
If you've got a heavy breather then you'll need more.
The way I check the volume is to hook up an inline gauge with a large cylinder. (quart)
Then measure.

True for carb engine with carb pump . . . original questions are about FI!

[deaner]

34-43 psi!

I meant that a 3-4 psi drop in pressure is about the limit. Not that it should have this much pressure.


tekenaar please be careful editing my post as you edited for something I wasn't trying to say. You are correct on the total pressure.

[tekenaar]

This is a good point. Volume is more important than most would think. Also check the change in pressure. This is important. From full throttle to no throttle and then back on the throttle (as in high rpm shifting), too much of a drop in pressure is bad. Should be between 3-4 psi drop in pressure. You could be starving for fuel at this point. The pump must be able to keep up.]

I meant that a 3-4 psi drop in pressure is about the limit. Not that it should have this much pressure.

tekenaar please be careful editing my post as you edited for something I wasn't trying to say. You are correct on the total pressure.

. . . point taken . . . but you're wrong about FI "full throttle to no throttle and then back on the throttle" regulated fuel pressure variations. I believe you're thinking in carburetor fuel volume terms, not designed-in varying FI fuel pressure . . .

FI fuel pressure will vary exactly as you say - full throttle (lowest vacuum), max ~43 psi; no throttle/engine braking (highest vacuum), min ~34 psi; back to full throttle (lowest vacuum), max ~43 psi - but with a range of up to 9psi pressure drop, more than twice yours stated . . . and exactly as Bosch designed it!

[deaner]

FI fuel pressure will vary exactly as you say - full throttle (lowest vacuum), max ~43 psi; no throttle/engine braking (highest vacuum), min ~34 psi; back to full throttle (lowest vacuum), max ~43 psi - but with a range of up to 9psi pressure drop, more than twice yours stated . . . and exactly as Bosch designed it!

I stand corrected. I went back and looked at the Bosh manual. You are correct that up 9 psi drop. Thanks for pointing that out.

[wrench459]

This is how I check fuel volume.
"T" the tester inline on the supply line.
With the engine running and a properly working/sized return.
You'll need to see atleast 1/2 pint in less than 15 seconds on a FI system.
I tend to forget most people don't have the tools that are availably to me.
Another way to check the fuel pump is by using a low current probe. The Bosch pump is different than most it has 10 commutators versus the normal eight found in most other cars. All you do is look at the sequence of the repeating pattern to tell the rpms of the pump. Is there a spec, nope, but I can you tell you from experience, it's around 5800 rpms. 8 amps, where the so-called other cars are 6 amps (high pressure)
Now the low pressure systems only pull 2 amps or so.
There's misinfo all around my post . . . Who can correct it ???
????????????
Sorry

[deaner]

There's miss info all around my post Who can correct it ???
????????????
Sorry

I would get a copy of the Bentley "Bosch Fuel Injection & Engine Management" Book as well as a copy of the factory Manual. Lot's of correct info there. There is also a trouble shooting and service section that would help. That way you have the info in front of you. HTH

[wrench459]

Believe me you wont pause
Most OEM'S dont release fuel volumes info
good luck....

[wrench459]

I need to correct this
I use current ramping of the fuel pump.
Still waiting for an old Bosch fuel pump to come into the shop.
Gene may I probe your 75 fuel pump..dang that sounds bad.