Aggressively Rough Idle / Driving

[Ol' Bessie's Keeper]

So I was scanning some old threads and saw it could be the fuel pressure regulator that's throwing it for a whack. Primed the rail four times before trying to cold start and she fired up after the first crank, versus taking a few times before actually starting. Still sounded like she was starving for fuel before starting either way.

Is this indictive of a bad FPR?

 

[macleanflood]

Is this indictive of a bad FPR?

Yes but use a fuel pressure gauge to confirm.

And check the pressure while it's running. And not just idling. You have to pull the whole fuel hat to replace the regulator...it sits on top...and it would suck if the pump is bad and you should have just replaced the entire assembly.

-Mac

 

[Ol' Bessie's Keeper]

Yes but use a fuel pressure gauge to confirm.

And check the pressure while it's running. And not just idling. You have to pull the whole fuel hat to replace the regulator...it sits on top...and it would suck if the pump is bad and you should have just replaced the entire assembly.

-Mac

Well unfortunately I'm at a loss then, she doesn't drive too well. She will take off, drive, very rough and poorly and die at stops. Any other alternative?

 

[macleanflood]

You sure you got the intake on all the way? It's got dowells and it's easy to not get seated correctly.

How about mice nesting in the exhaust over the last six months?

Have you repeated the compression test now that you've replaced the piston?

Your fuel is at least 9 months old, right? Drain the tank and get fresh gas.

Also possible that gas gummed up the injectors.

-Mac

 

[Ol' Bessie's Keeper]

You sure you got the intake on all the way? It's got dowells and it's easy to not get seated correctly.

How about mice nesting in the exhaust over the last six months?

Have you repeated the compression test now that you've replaced the piston?

Your fuel is at least 9 months old, right? Drain the tank and get fresh gas.

Also possible that gas gummed up the injectors.

-Mac

Yeah, let me re-tell my timeline (as well as some other info) so there is a clearer understanding of what's going on, haha

Currently at 188K miles; after sitting, brakes are fully locked up but brake booster holds vacuum, so figuring that out on top of her running rough.

May 2026 - Just did a full rebuild and put in four new pistons; compression all above 150. Also at this time, put in some new fuel so I'd say it's a mix of 90% new & 10% old. I'm pretty certain I absolutely made sure to match up the exhaust and intake manifolds up properly, torque sequence & all; will have time later today to see.

I suppose mice could have nested in the exhaust but do find it unlikely.

Finally on my day off, so gonna just check around under the hood for a bit and see if anything looks out of whack again.

 

[macleanflood]

Modern gas doesn't last more than two months tops before going bad enough to start causing issues.

-Mac

 

[Ol' Bessie's Keeper]

Here's a question: Would a failing brake booster cause enough vacuum issue to make it run terribly also? When I took her out for a maiden voyage back in May just to try to fill up the tank, she died every time while braking, and then it took a minute to crank and get her to start again. And back then, when I say braking...I mean TRYING to brake because the pedal was completely stiff the entire drive.

Now, the pedal presses down again (this issue fixed itself I guess), but it does have a very "spongey" noise when pressing the pedal down. Brakes work, but that noise is pretty noticeable.

 

[Rickyd]

Here's a question: Would a failing brake booster cause enough vacuum issue to make it run terribly also? When I took her out for a maiden voyage back in May just to try to fill up the tank, she died every time while braking, and then it took a minute to crank and get her to start again. And back then, when I say braking...I mean TRYING to brake because the pedal was completely stiff the entire drive.

Now, the pedal presses down again (this issue fixed itself I guess), but it does have a very "spongey" noise when pressing the pedal down. Brakes work, but that noise is pretty noticeable.

Plug the hose to your booster to rule it out

 

[macleanflood]

I'd think a vacuum leak would cause a high idle.

-Mac

 

[Ol' Bessie's Keeper]

Update:

New fuel - runs the same; rough. Still takes a few "primes" to the fuel rail to get it to fire on the second crank; still sounds starved.

Brake Booster - Pulled the hose, plugged; runs the same- no change!

Manifolds seem to be seated properly; I should still test for vacuum leaks around there since it does sound like a faint "squeal" of a vacuum leak is occurring somewhere within the engine bay.

That's about all I got.

 

[macleanflood]

What are your fuel trims I live ODB data?

-Mac

 

[Ol' Bessie's Keeper]

Short trim stays basically gets as low as -25% to -29% at idle. Put her under load around the block, the values stay within 0% to -10%. And then some of the time it would just not read a value, i.e. stayed at 0% with no flutuation.

 

[Steel City 06]

Modern gas doesn't last more than two months tops before going bad enough to start causing issues.

-Mac

Fun tidbit:

Higher octane fuels can be stored 2-3x longer without significant issues. Sunoco suggests 3 months for 87 but 9 months for 93:

https://www.sunoco.com/fuel-facts/post/are-high-octane-fuels-more-stable

That doesn't take into account winter vs summer blends though. Accidentally left a jerry can of winter gas in a hot truck bed once. Opened it up and it straight up erupted like diet coke and mentos on steroids because the gas can was well above boiling temperature for the winter blend gas...

 

[Steel City 06]

Could try dumping a bottle or two of Techron in the tank to dissolve gum deposits. It's the same chemical in Top Tier gasoline but just way more concentrated. I wouldn't bother with any other additives though.

 

[Steel City 06]

Short trim stays basically gets as low as -25% to -29% at idle. Put her under load around the block, the values stay within 0% to -10%. And then some of the time it would just not read a value, i.e. stayed at 0% with no flutuation.

If it's pulling fuel that hard, you're probably air starved, not gas starved. Meaning the restriction is somewhere in the intake or exhaust system and fuel is less likely to be the issue.

Unless you have unusually high fuel pressure, perhaps due to a plugged relief valve or something. (In that case it would be flowing way more fuel than it thinks it should be flowing.)

It could also be an error in the oxygen sensor system, but in that case you'd expect it to run okay in open loop.

 

[Steel City 06]

That said what we'd really need to know is total fuel trim to make that determination. Aka LTFT plus STFT. It's possible the ECM adapted to a vacuum leak over time, and now that it's gone, the LTFTs are completely wrong.

 

[Ol' Bessie's Keeper]

That said what we'd really need to know is total fuel trim to make that determination. Aka LTFT plus STFT. It's possible the ECM adapted to a vacuum leak over time, and now that it's gone, the LTFTs are completely wrong.

It wasn't popping any long trim numbers. Hadn't swung one way or the other the entire time of operation and idle.

 

[Steel City 06]

I'd be suspect of either low airflow (intake or exhaust) or high fuel flow.

Perhaps fuel pressure is too high or an injector is sticking or leaking. Any major differences between the bank 1 and bank 2 trims or are they close?

 

[Ol' Bessie's Keeper]

So this was at idle, having trouble retaining idle and sitting around ~500rpms. Bank 2 had no data readout, but swapped over to B1S2 and it shows that data. First time seeing any sort of LT data also.

 

[Steel City 06]

So there are two steps to closed loop fueling. Sensor 1 in each side is the first step, and it will use that shortly after startup. Once the catalytic converters have lit off after a few minutes of driving, it will switch to Sensor 2 on each bank and use those to control fueling.

If this has been running for a while, you'd expect to see something on Sensor 2 for each bank. If it's still running on Sensor 1 after several minutes, there may be an issue with the oxygen sensor or some other part of the emissions system.

These values are all Bank 1. Unless I missed something and this is a 4-cylinder engine, Bank 2 should also have some data.