stylo_guy

- 2007 LS
- 2.2 liter EcoTec engine
- auto trans
- about ~98k miles

So I've been chasing down this "random RPM stumble for a few seconds on the very first cold start of the day" problem for a while now, no luck though - been driving me crazy trying to figure out what is going on during that very "first" cold-start of the day No codes which doesn't help matters either. Been scanning and scanning and scanning it, chasing my tail not really getting anywhere since everything looks normal in the scan tool

Today I decided to stop by AutoZone to rent a fuel pressure gauge... just out of curiosity. Hooked up the fuel gauge connector right there in the parking lot for some testing.

With the key turned to "ON" I noticed it seemed to go to about 45 PSI... but then slowly dropped, it seemed to settle around 35-36 PSI after standing around/waiting around a bit staring at the fuel gauge. Went back in the car and took the key out, then waited a few minutes/afk, then reinserted the key and turned to "ON" just to make sure the first reading wasn't a fluke or anything like that. Turned the key to "ON" again and noticed this time it seemed to start out at 47-48 PSI... but then slowly dropped, it seemed to settle around 41ish PSI

While I was there I went ahead and started the car, and observed the fuel pressure seemed to hold steady at 60-61 PSI, didn't drop with a few minutes of me standing around staring at the gauge

Went back into the AutoZone and returned the loaner tester kit/got my refund, then came home to make this post

I pulled up the service history/service records for the vehicle, and apparently the previous owner already had a new fuel pump installed back in January 2022?? I just find it odd that a fuel pump could start to become weak/worn out after just 2 years...

Thoughts? Is it normal for fuel pressure to "slowly" drop with the key in the ON position (but the engine not running)? Or is something wrong somewhere?

PulpFriction

First, understand that if you turn the key to run, the fuel pump only runs for a few seconds, then stops if no start. Maybe only 2-3 seconds. I don't remember.

The fuel pump has pressure relief valve set at about 58-60 PSI, and a check valve. When all is well, the system should hold 58 psi or so for several minutes at least.

In your case, the system isn't holding pressure. It could be a faulty check valve, or a fuel leak. Is there any sign of fuel odor at any time? Please do answer this question. Fuel line leaks are very common in these cars, and inevitable in certain climates.

You might try turning the key to run for 2 seconds, turn off, and immediately repeat twice, to see if the fuel pressure builds to a normal level. If so, and if there is no fuel leak, likely the only problem is a faulty check valve, which is pretty harmless except for the annoyance at startup. You can reduce that by turning the key to run for 2 seconds, then off, then start as normal.

PSA - it's not necessary to turn the key until it starts. Just turn the key momentarily. That tells the computer to initiate the start, and it handles it from there.

stylo_guy

No fuel leaks or fuel odors that I have noticed. For example, when I get down and take a good look under the rear trunk area there are no wet spots on the ground or anything - completely dry under the fuel tank area

It just so happens my brother has a 2006 Chevy Cobalt, which is basically the same car as an HHR (same 2.2, same Delta frame/platform, etc)... and his did have a fuel leak in the rear, so I know what a fuel leak would smell like

...maybe it's possible it could be a tiny/pinhole leak somewhere, and it simply evaporates before it hits the ground or any smells develop? But I haven't looked that far into it yet. Just doesn't sound right to me that a "new" fuel pump could be going bad already when it was installed January 2022

I assume any fuel leaks would likely occur near the driver-side rear wheel? I assume that's where the fuel lines are located

donbrew

There is also a vapor pipe, if it gets a hole in it the tank cannot hold its pressure and fumes are released. (not the fuel pressure). There are 2 pipes: fuel and vapor. Most people think the vapor is a return.
If the key is on the purge and vent valves are active. After you turn the key off it should not lose more than 5 psi in one minute. (one minute seems like a long time when you are waiting)

stylo_guy

So do you mean turn the key to "ON" for 2 seconds, turn key off, then turn key all the way to start as usual? Just asking so there's no confusion - if you mean actually turn the car on, turn the key off, and then turn the car back on again that's a different thing

I think I'll try what you mentioned tomorrow morning and see how it behaves on that very "first" cold-start of the morning. Right now it's still a hot engine since I've made a few trips to some stores, so it wouldn't act up

Typically what happens on the very "first" cold-start of the day is as follows:

1. the starter turns over, but seems to crank longer than usual before the engine actually fires up
2. during the very first 1-3 seconds of the engine coming on, it seems the RPMs "stumble" noticeably - they go up and down briefly and engine acts as if it's not getting enough gas (has the distinctive "sputter" of a car starving for gas)
3. if the headlights happen to be on, the headlights and dashlights will dim as the RPMs are "stumbling" up and down (the car may also have a physical vibration as the engine idles rough during these early moments)
4. around 6-7 seconds later the RPMs will stabilize a bit and then the RPM needle will idle steady
5. in all subsequent startups throughout the day (such as coming out of the grocery store or a return trip from work), the car will fire right up... turn the key and engine starts immediately, there is no "lag time" (from turning the key to engine firing up) like with the very first cold-start

The one common denominator for the weird "random RPM stumble for a few seconds on the very first cold-start" symptom seems to be: the engine has been off for an extended period of time, such as overnight

All the hot-starts are smooth and engine fires up instantly without any "lag"

RJ_RS_SS_350

Turn key to ON for 3 seconds, turn key off. Turn key to ON for 3 seconds, then turn to START and release the key.

donbrew

It is in your shop manual.

stylo_guy

So is that the test for a bad check valve?

I'll try it tomorrow morning (for cold-start condition) and see how it starts

stylo_guy

Hmmmm, so I cycled the key twice for the fuel pump prime and it did seem to start smoother

But reading about this "check valve", supposedly it's part of the fuel pump/built-in?

Really don't feel like spending big $$$$$ for an ENTIRE fuel pump replacement, is it possible to just buy an "external" check valve and install it somewhere on one of the fuel lines? Perhaps install it on the fuel filter?

Just brainstorming a bit here, and first thing that came to mind is some sort of "external" check valve since that's the cheapest thing obviously. Looking at Amazon these "fuel pressure check valves" seem to sell for $10-$30

Thoughts? Has anyone installed one?

firemangeorge

Uhhh. No fuel filter on a HHR.