Engine Hiccup at full throttle

AeroCoupe · Jun 5, 2026

But the ECU can only correct so far so the fuel pressure can be increased to a point where it can no longer correct.

keel · Jun 5, 2026

Okay I got my adjustable FPR installed, it's the one that still has vacuum attached to lower fuel pressure at idle. I have it set to 45 PSI with the vacuum line unplugged so at WOT I should be seeing 45 PSI of fuel pressure and that should help me figure out if I'm running lean.

With the vacuum line attached at idle, I see about 39 PSI, but when I rev the engine in neutral the fuel pressure drops. If I disconnect vacuum and rev the engine, the fuel pressure stays stable at 45 PSI. This would suggest that I'm pulling more vacuum as I rev the engine, which makes no sense.

Anyone have an explanation for this?

AeroCoupe · Jun 5, 2026

That’s the way it’s supposed to work. The FPR is keeping a constant pressure at the injector. Vacuum is negative pressure so the fuel pressure drops as vacuum increases to keep the pressure at the injector the same.

Vacuum off - 45 psi
Vacuum attached - 39 psi (idle vacuum is probably -6 psi)

With Jo vacuum reference the FPR will keep the pressure at set point but at idle the pressure at the injector will be the set point plus the vacuum so 51 psi.

Make sense?

keel · Jun 5, 2026

I understand that part, but I'm seeing the fuel pressure drop to 30-35 PSI when I increase the engine RPM, and then goes back up to 39 PSI at idle. Which seems backwards to me because I thought the manifold would pull less vacuum as you add throttle.

Usually this would indicate that the pump isn't flowing enough but I know it's not a fuel flow issue because fuel pressure stays stable at 45 PSI while revving the engine as long as the vacuum line is disconnected.

AeroCoupe · Jun 5, 2026

Holy hell you are correct. At idle it will be less than the set pressure as the vacuum is at its highest. As you give it the beans it will increase to set pressure. I’m a few cold ones in and enjoying the cooler weather before a front hits on the back porch.

Do you have a gauge you can connect to a vacuum source so you can see what the vacuum pressure is?

keel · Jun 5, 2026

Actually I think there’s a way to rig up my fuel pressure gauge to read vacuum I’ll try that tomorrow.

AeroCoupe · Jun 6, 2026

Remember, vacuum is negative and in inches of Hg. 1” of Hg is roughly equal to 0.5 psi but again that will read negative so I’m not sure a fuel pressure gauge will work. You can get a cheap vacuum gauge at most parts stores or if you have Harbor Freight they have them.

keel · Jun 10, 2026

Weather has been bad so I haven't been able to make a ton of progress. I didn't check the vacuum but I went out for a drive with the fuel pressure set to 45 PSI and the car felt great, pulled hard and no misfires. Dropped the fuel pressure back down to 40 PSI and it felt pretty much exactly the same. So I'm not sure if bumping up the fuel pressure had any effect at all.

However, I disconnected the battery when I was installing the adjustable FPR and as a byproduct I reset the ECU. I think this is what fixed my issue, but I'm not sure why so if anyone can weigh in that would be great.

Noobz347 · Jun 10, 2026

I had to read that a couple of times. LoL

This part:

keel said:
I didn't check the vacuum but I went out for a drive with the fuel pressure set to 45 PSI and the car felt great, pulled hard and no misfires.

Did not jive with this part:

keel said:
Dropped the fuel pressure back down to 40 PSI and it felt pretty much exactly the same. So I'm not sure if bumping up the fuel pressure had any effect at all.

keel said:
However, I disconnected the battery when I was installing the adjustable FPR and as a byproduct I reset the ECU. I think this is what fixed my issue, but I'm not sure why so if anyone can weigh in that would be great.

Set the fuel pressure to stock, reset the ECU and drive for a while. I mean... Drive it a few times over a week and see how it acts. Post up the behavior.

If it's fixed with an ECU reset then WIN!!!

If it acts up, bump the FP, reset the ECU and drive it another week.

keel · Jun 10, 2026

Noobz347 said:
Set the fuel pressure to stock, reset the ECU and drive for a while. I mean... Drive it a few times over a week and see how it acts. Post up the behavior.

I've got it at 40 PSI with vacuum disconnected and 35ish with vacuum. That should work right? I think there is supposed to be a bigger difference but with the cam I'm not pulling as much vacuum at idle compared to stock.

Noobz347 · Jun 10, 2026

Just run it and see how it acts.

Engine Hiccup at full throttle

AeroCoupe

lube between the nut and the face. I know my lubes

keel

Active Member

AeroCoupe

lube between the nut and the face. I know my lubes

keel

Active Member

AeroCoupe

lube between the nut and the face. I know my lubes

keel

Active Member

AeroCoupe

lube between the nut and the face. I know my lubes

keel

Active Member

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

keel

Active Member

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

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