Can somebody please advise me where the fuel pressure regulator is on a 05 4.2L Trailblazer? I cannot seem to find it. Stealer wants $3800 to replace the head for a intermintent random misfire. Sometimes runs fine, sometimes misses so bad it dies...even while driving. They say burned exhaust valves, I say NOT! Do the purge valves ever go bad and cause this? I'll replace dang near everything before I replace the head.
Trailblazer fuel pressure regulator location
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"The fuel pressure regulator (2) is contained in the fuel sender assembly. The fuel pressure regulator is a diaphragm relief valve. The diaphragm has fuel pressure on one side and regulator spring pressure on the other side. The fuel pressure regulator is not vacuum biased. Fuel pressure is controlled by a pressure balance across the regulator. The fuel system pressure is constant."
My pressure swings wildly at idle from about 55 to 62 psi. Very fast cycle back and forth, maybe 6 times a second. Isn't the valve you describe just a pressure relief valve like an oil pump has, to relief excess pressure before it enters the system?
JTK
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2005+ Trailblazers used a pulse-width modulated fuel pump to regulate fuel pressure. It doesn't have a fuel rail mounted press reg with a return to the tank.
The #'s you posted may be correct. You're positive it's a fuel pressure problem?
Any issues with that would require replacement of the FSU I'd think.
Have you had a compression test done? Any Vortec/Atlas 4.2L valve seat issue I've ever heard of had low compression as well.
Coils can be an issue as is the MAF sensor for random misfires.
The #'s you posted may be correct. You're positive it's a fuel pressure problem?
Any issues with that would require replacement of the FSU I'd think.
Have you had a compression test done? Any Vortec/Atlas 4.2L valve seat issue I've ever heard of had low compression as well.
Coils can be an issue as is the MAF sensor for random misfires.
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No I am not positive it is a fuel pressure problem. When watching the scan tool STFT it moves from positive 5 to about negative 7. At neg 7 it starts to misfire then swings back the other way. LTFT shows neg 3 to about neg 7 different times I have noticed. #1 O2 sensor cycles back and forth between 800+ and 100- as I think it shoulde. #2 O2 sensor stays high between about 600 and 900. The big problem is it can lose all power when driving/accelerating, standing on the throttle clears it up and it runs OK for a while. When it loses all power while driving it acts like it is running on about 2 cylinders and the check engine light flashes, after standing on it and it clearing up the check engine light is steady again.
What CEL codes are you getting?
P0300. When I reset it, sometimes it will come back in a day, sometimes a month. The problem is very intermitant.
Originally Posted By: JTK
2005+ Trailblazers used a pulse-width modulated fuel pump to regulate fuel pressure. It doesn't have a fuel rail mounted press reg with a return to the tank.
The #'s you posted may be correct. You're positive it's a fuel pressure problem?
Any issues with that would require replacement of the FSU I'd think.
Have you had a compression test done? Any Vortec/Atlas 4.2L valve seat issue I've ever heard of had low compression as well.
Coils can be an issue as is the MAF sensor for random misfires.
Trailblazers do not use a PWM controlled fuel pump. It is a returnless system, but a standard diaphram pressure regulator is part of the fuel sending unit.
It's also not a MAF problem; the 4.2L didn't get a MAF sensor until 2006.
As for replacing the head, I wouldn't let anybody replace it until I had cylinder leakdown results in hand. Misfire caused by a valve seat issues is most always felt only at idle. It's pretty unmistakable.
2005+ Trailblazers used a pulse-width modulated fuel pump to regulate fuel pressure. It doesn't have a fuel rail mounted press reg with a return to the tank.
The #'s you posted may be correct. You're positive it's a fuel pressure problem?
Any issues with that would require replacement of the FSU I'd think.
Have you had a compression test done? Any Vortec/Atlas 4.2L valve seat issue I've ever heard of had low compression as well.
Coils can be an issue as is the MAF sensor for random misfires.
Trailblazers do not use a PWM controlled fuel pump. It is a returnless system, but a standard diaphram pressure regulator is part of the fuel sending unit.
It's also not a MAF problem; the 4.2L didn't get a MAF sensor until 2006.
As for replacing the head, I wouldn't let anybody replace it until I had cylinder leakdown results in hand. Misfire caused by a valve seat issues is most always felt only at idle. It's pretty unmistakable.
It would be more cost effective to remove the head, and if the valves are burnt, replacing them and having a valve job done. I couldn't imagine it costing more than $1000-1500.00 to do this job.
I retested the fuel pressure, 60 psi steady. Vehicle right now runs smooth but will not rev over 3500 rpm in neutral with it floored. Smooth, but slow to increase RPM, no smoke. Very steady 16.5 inches of vacuum at idle.
Now I removed my #1 O2 sensor and replaced it with a 0 - 10 psi pressure guage. No measurable back pressure there.
Rev limiter? I don't know, driving I used to be able to rev over 5000 rpm.
valve seats and guides are pretty common on these trucks. would recommend getting a second opinion and having a compression test ran. if the pcm detects a weak cylinder it will shut it down completely. and the loss of compression can be very small for it to shut the cylinder down. 3800 seems kinda high.
There is a rev limiter that prevents the engine revs from going more than 3,000-3,500 RPM when the vehicle is in park or neutral. It will go well past that when driving, but it limits free-revving of the engine.
Originally Posted By: JasonC
It would be more cost effective to remove the head, and if the valves are burnt, replacing them and having a valve job done. I couldn't imagine it costing more than $1000-1500.00 to do this job.
New valves and a valve job won't cure the problem. The issue is the valve seats, not the valves. You'd have to pay the machine shop to cut out all the existing intake valve seats (12 of them) and press in new seats, along with replacing the valves.
At that point, it's cheaper to just get a remanufactured head from GM that has already has the updated seats installed.
The valve seat issue is realitvly well known amongst the Atlas L6, L5, and L4 engines. GM has written a couple bulletins on the concern and even released a special tool to hold the timing chain in place so the head can be removed without the need to re-time the engine. This eliminates the need to take apart the front of the engine, saving probably 3+ hours of disassembly and assembly.
It would be more cost effective to remove the head, and if the valves are burnt, replacing them and having a valve job done. I couldn't imagine it costing more than $1000-1500.00 to do this job.
New valves and a valve job won't cure the problem. The issue is the valve seats, not the valves. You'd have to pay the machine shop to cut out all the existing intake valve seats (12 of them) and press in new seats, along with replacing the valves.
At that point, it's cheaper to just get a remanufactured head from GM that has already has the updated seats installed.
The valve seat issue is realitvly well known amongst the Atlas L6, L5, and L4 engines. GM has written a couple bulletins on the concern and even released a special tool to hold the timing chain in place so the head can be removed without the need to re-time the engine. This eliminates the need to take apart the front of the engine, saving probably 3+ hours of disassembly and assembly.
Yeah, but leaking valves would not cause sudden complete loss of power when just driving down the road.
Originally Posted By: another Todd
Yeah, but leaking valves would not cause sudden complete loss of power when just driving down the road.
pcm detects a weak cylinder and completely shuts it down to prevent damage to cat. converter.
Yeah, but leaking valves would not cause sudden complete loss of power when just driving down the road.
pcm detects a weak cylinder and completely shuts it down to prevent damage to cat. converter.
Originally Posted By: MrHorspwer
Originally Posted By: JasonC
It would be more cost effective to remove the head, and if the valves are burnt, replacing them and having a valve job done. I couldn't imagine it costing more than $1000-1500.00 to do this job.
New valves and a valve job won't cure the problem. The issue is the valve seats, not the valves. You'd have to pay the machine shop to cut out all the existing intake valve seats (12 of them) and press in new seats, along with replacing the valves.
At that point, it's cheaper to just get a remanufactured head from GM that has already has the updated seats installed.
The valve seat issue is realitvly well known amongst the Atlas L6, L5, and L4 engines. GM has written a couple bulletins on the concern and even released a special tool to hold the timing chain in place so the head can be removed without the need to re-time the engine. This eliminates the need to take apart the front of the engine, saving probably 3+ hours of disassembly and assembly.
well said, have had a few role through the shop.
Originally Posted By: JasonC
It would be more cost effective to remove the head, and if the valves are burnt, replacing them and having a valve job done. I couldn't imagine it costing more than $1000-1500.00 to do this job.
New valves and a valve job won't cure the problem. The issue is the valve seats, not the valves. You'd have to pay the machine shop to cut out all the existing intake valve seats (12 of them) and press in new seats, along with replacing the valves.
At that point, it's cheaper to just get a remanufactured head from GM that has already has the updated seats installed.
The valve seat issue is realitvly well known amongst the Atlas L6, L5, and L4 engines. GM has written a couple bulletins on the concern and even released a special tool to hold the timing chain in place so the head can be removed without the need to re-time the engine. This eliminates the need to take apart the front of the engine, saving probably 3+ hours of disassembly and assembly.
well said, have had a few role through the shop.
Snapcrackle, I get what you are saying, but having driven the vehicle when it loses power there is no way it is dropping one or even two cylinders. It's much worse than that and only for a few seconds (3-6) then it's back up to power. Once on the freewaqy at 65 MPH steady it just died like the ignition switch was shut off. After coming to a stop, it started right back up without further issues that day. Not sure if that is related to current problem though.
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