Stumbling after start up

[Wgesnerjr]

Well, it happened again last evening. Hesitating and bogging down. Only after opening it up did it smooth out. Decided to NOT concentrate on it for a while and moved on to other things. Fixed a hood alignment issue and checked fluids. Found the trans was over filled.

This morning I drained out some trans fluid and topped off to correct level. Also checked the belt. No shakey tensioner and belt on all the ribs like normal. Ran a few errands but no issues that I could tell.

Still on a wait and see.

Thinking it might be a lazy throttle body. Checked prices, had a heart attack and decided to ignore for a while.

[62chevy302]

I feel like 41 psi is a little low. I think it should be putting out around 60 with vacuum pipe off the regulator and low 52 running normal.

[lutronjim]

Spec for my LC9 5.3 was 50 PSI

[mongocanfly]

I didn't catch where he stated fuel pressure...I agree...41psi is low....mine idles at 56psi

[Wgesnerjr]

I thought it was low, but other times it runs well. Again, this issue does not happen every time, only occasionally and right after start up. (both cold and hot) Once you run it up in first gear, the smooths out and runs great for the rest of the ride. I would think if the PSI being low was the cause, the issue would ALWAYS be there, or at least come back during that ride, right?

Example: Drove it to work Monday and no issues starting up in morning. Drove entire 20 miles with great power. After work, did have the issue. Noticed it pulling out of parking deck. Again, ran it up at half throttle to the 1-2 shift and it cleared out like always. Ran great after that in stop and go traffic for next 1.5 hours.

Back when I tested the pressure, the problem was not happening. So it was running fine with the 41psi. (i know, weird, right??)

I want to try a new regulator but the price is high to just "see" if this fixes the issue. Next time it's happening, I may try stopping and pulling the vac source. Then try and drive at the higher pressure to see if the issue stops. Would there be harm in trying this??? (driving with the regulator disconnected for a few miles?)

Also of note: I have used the underhood fuse box from the Tahoe. So the electrical source for the pump is the stock relay. Truck shows good voltage at 13 or so volts during driving and initial start up.

Thanks to all for helping me sort out an annoying problem.

[62chevy302]

If you pull the fuel pressure regulator hose off it will have a vacuum leak. As long as you block that off it will be ok

[Wgesnerjr]

Drove it to get a Christmas tree and a couple of other places. Happened twice, both times I could not stop to try and pull the vac line on the Fuel Pressure Regulator.

Did find that the power steering fluid has taken on some water as one point. I had filled it with new fluid when I first got the truck on the road in April. Now the fluid is gray brown? Will be changing that this weekend.

I will need to drive it to work this coming Thursday and Friday, so I'm sure we will get some testing in then.

[solidaxel]

Same issue about 15% of the time
Blip the throttle in neutral and it clears up.....only at start up

[Wgesnerjr]

Quick Update:
Worked on flushing the power steering fluid this weekend and that went well. Still a slight shutter when turning left. Will focus on that later.

Drove it and got it to act up twice. Both times I pulled the vac line off the fuel pressure regulator and it seemed to clear up by the time I got back in the truck and drove. One time I plugged the vac line, one time did not. Both times the problem seemed to have cleared up. One thing we noticed is the exhaust smelled very rich while it was acting up. Once the vac line was pulled, the smell was gone.

SO please check my thinking here. When the truck is at idle, vac is high. High vac signal on fuel pressure regulator pulls the pressure DOWN. Vice Versa- When throttle is open, vac is low. Low vac signal on fuel pressure regulator increases pressure UP. Is this correct?

If the above is correct thinking, would a bad fuel pressure regulator cause a rich condition and the stumble? Would running it up in first gear clear this out?

[Wgesnerjr]

I was looking around the internet for a decently priced regulator and found this video.

https://www.1aauto.com/fuel-pressure...ALLATION_VIDEO

Shows how to replace it, but in doing so, shows there is a screen behind the regulator. I'm now wondering if this screen could be clogged. This will be the first thing I check out sunday.

[Wgesnerjr]

The suspected filter is the one all the way on the right hand side in the pic. I didn't even know that was back behind the Regulator.

On a side note: I have developed an oil lead on the drivers side just above the oil filter. I did not use the part above the oil filter for my oil pressure sending unit. (held in with two bolts, can tap into for oil pressure sending unit) However, I think this is the part that has developed the leak. Can anyone tell me the official name of this part so I can search out a new gasket?

Thanks to all.

[BR3W CITY]

Its called an OFAC, or oil cooler blockoff. Gasket should only be a couple bucks.

[Wgesnerjr]

Got it!!
6-9 bucks locally.

Thanks.

[Wgesnerjr]

So I finally broke down and bought a new Fuel Pressure Regulator and installed this weekend between rainstorms.

Results: Same as before. Still stumbling and stuttering on start up. Only happens occasionally, not every time. Quick run up in first gear and it clears right up. No more problems during the entire drive. Plenty of power and response.

Fuel pressure pretty much the same as before 44 with vacuum hooked up, 55 without. I imagine if it was pressure related, it would happen all the time.

Wondering now if injectors could be clogged. engine has 179K on it.

[Wgesnerjr]

Decided to test the fuel pump today. Hooked up my fuel pressure gauge in place of the fuel filter. With key on, the pressure went over a 100 as the fuel pump went off for the first 5 seconds, then went down to 80 and held for over 30 mins.

Thought I should test on the other side of the filter, just to see if filter was clogged. Did the same thing got same readings. Gauge pegged for 5 seconds the held 80.

Why would I have over a 100 at the fuel filter and only 44-55 at the fuel rail test point? Only fuel lines between filter and fuel rail. Remember, the stock regulator was changed already with little effect. Cleaned TB. 12.1 volts to fuel pump. No mil. Only thing I found was a bad connection going to evap solenoid. Could this be the issue?

Starting to think this might not be fuel related. Heated O2? PCM program maybe?

[mongocanfly]

if your reading fuel pressure at the rail Schrader valve that should be the regulated fuel pressure...that's why it would be lower than the pump pressure

[Wgesnerjr]

Some have been telling me the pressure at the rail is low and could be contributing to the issue. The whole swap is basically stock, so I'm still wondering if the pressure is truly low enough to effect the stumbling and hesitation?

[mongocanfly]

that is low ...compared to mine..mine is about 58-60psi

[nsb29]

I don’t know that much about these LS motors but it does not seem that gunning the engine would change anything to do with the fuel pump or pressure does this truck have DOD just a thought how about oil Pressure ?

[Wgesnerjr]

No DOD, oil pressure is good.
Also found this thread on another site.

https://www.tahoeyukonforum.com/thre...-engine.87057/

States for my VIN code "Z - Flex Fuel" pressure should be 48 - 54 psi.

Regular fuel engines (55-62 psi) VIN: (V, T,U)

[homemade87]

Originally Posted by Wgesnerjr

No DOD, oil pressure is good.
Also found this thread on another site.

https://www.tahoeyukonforum.com/thre...-engine.87057/

States for my VIN code "Z - Flex Fuel" pressure should be 48 - 54 psi.

Regular fuel engines (55-62 psi) VIN: (V, T,U)

The return style fuel rail do run at a lower pressure . Your pressure seems to be normal . The return-less fuel systems do run at 58 psi which is not what you have .

The only thing that is hard to detect is a fuel pump with a intermittent problem .

[jjzepplin]

Maybe next time it stumbles try NOT clearing it out and do some checking. Almost sound like a spark plug or wires issue. I would not think that revving the engine would clear up a fuel pump pressure issue. I could be wrong though. Could it be that your voltage output is low for a moment giving your pump low pressure and revving bumps it back up and clears it all out?

[Wgesnerjr]

Between work, the holidays and a back injury, I have been testing out how long this problem occurs without clearing it out. It takes about 30 sec to 1 1/2 mins then it will clear itself.

I have also have an OBDII bluetooth scanner. After start up, it takes some time to start sending data to my phone. But I finally got some data while issue was happening. First two screenshots are of the O2 sensors when its stumbling at idle. The next two are as the problem clears itself.

O2 2x1 would be Bank 2 and it looks like it may hold some clues. This is all happening under 150 degrees. I think it may explain the rich smell. Which side of the engine is bank 2?

[71cadc10]

if you didn't get an answer yet, bank 1 is driver side, bank 2 passenger

cylinder #1 is your front cylinder, driver side. so that is bank 1 for computer speak

Bank 2, sensor 1, would be the front O2 (pre-cat). sensor 2 would be the rear O2 after the cat

I'd replace the front O2 on passenger side, appears that it is not switching. If that doesn't fix it, check wiring to that sensor...they can get burned by the back of the head, or exhaust if get too close. Sometimes you need to peel the braid or convolute tubing off the outside to see the condition of the wires.
If the wires are bad....replace with good high temp automotive wire (150*C rated preferred) and use silver solder...not splices crimped with hand tool

[Wgesnerjr]

Thank you for the info on the Banks

I looked at the O2 wiring this weekend and it appeared good but will check again.