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Engine sputtering in gear

I have a 1999 Pontiac Montana with ~254,000 miles.

After driving it about 60 miles, the engine starter sputtering if I took it above 2k rpm, but only if it’s in gear. If it’s in park, the engine runs fine.

If I let it rest for a few minutes (either off or running in park), it’ll act normal again for a few minutes. It seems like eventually it gets worse and sputters at 1.5k rpm and I’ll need to pull over.

The idle has been sometimes rough or not without any clear pattern. It’s not any worse now. There’s been a P1404 code for the EGR valve being stuck open. But I replaced the EGR valve and tested the connections and that’s still there.

There’s no new codes though.

The most likely candidate sounds like the fuel pump, based on the description of what happens when it starts to fail.

I thought that an engine running a 3k rpm in park would need the same fuel pressure as an engine driving a car at 3k rpm, but I guess not?

Unfortunately, I’m not sure this is something I can do myself, simply because I don’t have anywhere to put 20 gallons of gas.

You’d have to hook up a fuel pressure gauge and check it driving. It could be lots of things though. The EGR code can take a while to come back after you reset it.

First off, I think you’ve still got an EGR problem of some kind. Not saying that’s the cause of the high speed sputtering, but it could be. You say your idle is not so good, and that’s likely due to the EGR problem remaining. Replacing the valve wouldn’t fix an EGR problem that was caused by the control system telling the valve to open when in fact it shouldn’t. A constricted vacuum hose could cause that to happen, among other things. EGR systems during that era were quite complicated. There’s usually multiple electronic controlled vacuum switches involved with the control function, as well as temperate sensors.

For the high speed sputtering problem, assuming all the routine engine maintenance is up to date according to the owner’s manual schedule, especially fuel filter, engine air filter, and spark plugs, I’d start by reading out all the stored diagnostic codes and go from there. A partially plugged cat should be considered too.

You are mostly correct. The fuel pressure is held constant independent of engine rpm, and the amount of fuel injected depends only on the duration of time the injector is pulsed open. High fuel demands result in a higher % of time the injectors are pulsed open. The actual fuel pressure at the rail (if you measured it referenced to ambient air pressure) does vary a little with engine load b/c the pressure that affects fuel flow is the difference between the fuel rail and the intake manifold. So when the intake manifold vacuum level is high (low pressure), the fuel rail pressure will be a little lower, and when the intake manifold vacuum is low (near ambient air pressure), the fuel rail pressure will be a little higher to compensate.

The EGR valve is electronic, not vacuum operated. Unless you’re talking about a different vacuum hose?

My understanding is that the code is set off when the pintle position sensor differs from the input signal from the PCM. Why that would be is mysterious to me. Very occasionally the check engine light disappears, but never for very long.

I think it was recommended to me before to do something that force the PCM to relearn the EGR signal values, but that didn’t seem to help.

I replaced the fuel filter, air filter, spark plug, and spark plug cables when I first bought it 5000 miles ago.

Wouldn’t that mean the pintle is sticking?

It’s unlikely that it’s sticking. The original one looked clean, but I cleaned it anyways. Same code. I bought a new one, same code.

Check for an exhaust restriction, excessive back pressure can cause poor performance and hold the EGR valve open.

That makes a lot of sense. I don’t have any way to check at the moment, but I did just notice that the very end of the exhaust looks crushed.

At least I’m supposing it’s not meant to be like this.

That bent tail pipe is not a significant restriction, a clogged or damaged catalytic converter could be the problem.

Does the exhaust sound normal at 2,500 RPMs or does it hiss?

It sounds normal to me.

I read that you can measure the temperature of the pipes before and after the catalytic converter to determine if it’s clogged. After warming up, it measures about 80 degrees warmer on the output side.

Should I take that as proof it’s fine?

I took it somewhere to get the fuel pressure tested. He said it was 10 psi lower than it should be, but probably not the problem.

Something I did find out is that one of the cylinders is misfiring (it looked like all of them were misfiring sometimes, but it was much worse on 2), corresponding to how rough the engine in running at idle. I’m not sure whether a misfire could cause the engine to sputter like it does though.

The spark plugs and cables are pretty new. I pulled the spark plug, and it looked normal to me.

I have an inductive tachometer on hand, I was thinking that might be a reasonable way to test of the coil pack is functioning correctly? It looks like the suspect one is newer than the rest though.

If it’s not the coil pack, I think the next thing to be suspicious of is the fuel injector?

Also, I assume I can switch up the coil packs?

Yep, and spark plugs, too, to take those out of the equation.

Pay attention to the coil mounting surfaces and remove any grime or corrosion.

10 pounds low on the rail fuel pressure is too low. Spec is often around 50, so 10 psi low is 20% low. Too much to ignore. That could cause misfires and rough idling. Perhaps your mechanic is thinking his fuel pressure gauge is reading a little on the low side, that’s possible. But if it is really 10 psi low, that’s got to be corrected before investigating other potential causes. It’s like if you home’s voltage is only 90 volts when it should be 115, no since taking the AC apart to see what’s wrong with it, not until the home’s power line voltage is up to spec.

If your car uses a fuel pressure regulator located at the end of the fuel rail, with the engine off, remove the vacuum hose connected to it. Look inside that hose. If there’s any gasoline at all inside that hose, the fuel pressure regulator must be replaced. Another simple test of the regulator is to compare the fuel pressure at idle to what it does when you briefly bump the rpm to 1500-2000. It should go up, maybe 10 psi or so, at least briefly. If it doesn’t, that’s another sign the fuel pressure regulator is faulty. I don’t think the engine compartment fuel pressure regulators are overly expensive to replace, so you might take a flyer and just replace it with a new one, see what happens.

I absolutely agree. Fuel injectors require much higher pressure to operate properly.

You know how if you squeeze the Windex trigger fast and hard you get a nice fine spray, but if you squeeze it slowly and with little pressure it drips and burbles out of the nozzle? Your fuel injectors are dripping and burbling. The spray needs to be a nice, fine spray for the fuel to burn properly.

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Okay. I guess I’ll try replacing the fuel regulator first, since that’s relatively inexpensive and easy.

I didn’t see any fuel in the vacuum hose. Should anything odd happen if the vacuum hose is disconnected with the engine running?

I found a program that can count the misfires with my obd2 reader. The odd thing is that most of them are on 1 – on the mechanic’s device they were mostly on 2. I switched all the coil packs around, but it didn’t make a difference.

All of the cylinders are misfiring though at idle, but they mostly don’t once the engine revs up.

But I guess the first thing to do is fix the fuel pressure.

The vacuum hose from the fuel pressure regulator to the intake manifold is there to reference the rail fuel pressure to the intake manifold, rather than to atmospheric pressure, b/c that’s where the injectors inject the gas to. The amount of gas injected for a given injector pulse duration is directly proportional to the rail pressure minus the intake manifold pressure.

As an example, say the desired pressure difference between the rail and the intake manifold is 60 psi. All the engine computer software assumes that. At idle the intake manifold is -10 psi (= a vacuum of 20 mm Hg), so the rail is regulated to 50 psi. If you removed the vacuum hose then the reference would be 0 psi (atmospheric pressure), and the fuel rail should go up to 60 psi. If you did that when the engine was running at idle the fuel mixture would get richer that it should, and the O2 sensor would detect it , which would result in a negative fuel trim showing up on the scan tool.