Trailblazer fuel pressure regulator location

[oldmaninsc]

Originally Posted By: MrHorspwer
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.

Well then you need to notify the people that wrote the factory service manual. They changed the fuel pump design in 2005.

Personally I'd try replacing the ignition switch. They are a know problem on these vehicles and much cheaper/easier than anything else.

 

[another Todd]

I think I found the problem but will have to drive it a couple weeks to make sure it doesn't return.

Short story---slightly loose and slightly corroded positive battery terminal.

Long story---It happenened in my driveway after numerous start and stops of the engine...the battery went dead. I charged it and load tested it, load tested fine at 400 amps. Mind you, charging and load testing was done on the battery bolts (side post) not on the cable. Thinking back, I recalled once having a vehicle that had a similar problem where the battery went dead but load tested fine, no engine off voltage draws, and alternator working fine. The problem was the corrosion caused the connection to ask like a diode, starting amps leaving the battery was OK but very little amps could go back into the battery to charge it, so if you started it and ran it for a while the battery would "trickle" charge. A lot of short starts and stops would produce a dead battery. Remembering this, I decided to check/clean the terminals. Negative was tight and clean, positive was about finger tight and had a slight layer of corrosion on the prongs that dig into the side post lead. I cleaned all of the corrosion and reassembled. The vehicle now runs better then it has in a long time. In all the time I spent screwing around with this problem, I re-learned (or remembered) the number one rule when diagnosing intermittant problems---check the terminals first! If this does in fact turn out to be the fix, I am going back to the stealership and request a refund of my $160 dianosis charge they charged me just to tell me they had too replace the cylinder head to fix it ($3800).

 

[JTK]

Originally Posted By: MrHorspwer

It's also not a MAF problem; the 4.2L didn't get a MAF sensor until 2006.


My 2005 most certainly had a MAF sensor. Late model year possibly. I do know that earlier models didn't have a MAF and relied on a MAP sensor.

I'll take your word for it on the PWM'd FSU if your more familiar than I. I used to participate quite often on a Trailblazer/ Vortec/Atlas 4.2L board and thought that was the case.

Joel

 

[snapcrackle12]

Todd, let us know it that was the cause of the problem, voltage issues can cause havoc on late model vehicles. I hope that was all it was. Gotta be careful with stealerships the tend to jump the gun and/or stick it to the customer. I had a customer earlier this week take their 02 jag x-type to the dealer and get and estimate for 2500 for lean code. The dealer wanted to replace intake gasket for lean code and brake booster due to poor assist and a few other things. I get it and intake gasket is good, brake booster is good. The vacuum hose between the two was melted on the backside. Fixed it and fuel trims returned to normal and brake assist work flawlessly. Fixed for 250 bucks. Guess the jag tech was a little too hungry, I will eat good tonight and tomorrow.....lol

 

[another Todd]

Well that wasn't it. Today it was missing steadily, code was for cyl 3 misfire. I disconnecte # 3 coil while it was running, no difference (that perticular coil was replaced about a year ago)I disconnected all other coils one at a time and they were all firing (idle worsened when disconnected. So I switched #3 and #4 coils. Now they all are firing. Any Ideas? I am going to switch them back and see what happens.

 

[another Todd]

OK I switched them back and #3 is dead again. Could this be a bad plug that this coil can't fire but the other one can?

 

[another Todd]

There are currently iridiums in it, do platinum or double platinums take less current to fire?

 

[another Todd]

Should I try a hotter plug?

 

[another Todd]

I pulled the two plugs, they were dbl platinums, I replaced them both and now it is running normal. I put the coil that wouldn't fire the #3 cylinder back in number 3 and it is now firing. We will see.

 

[another Todd]

Problem back. If I had hair I'd pull it out. Tonite I went to take the wife to dinner. It cranked and cranked for about 20 seconds, just as the battery was going dead it started and had a definate miss, sounded like one cylinder out. Will play with the coils again tomorrow. Could a PCM be causing all this?

 

[snapcrackle12]

take it to a good known independent shop and have them check it.

 

[Nick R]

It's the coils. Our 4.6L Expedition had this problem, when one or two of the coil packs went bad.

My mom's co-worker had a 2004 Trailblazer with the I-6 and had the same problem as you, and it was ignition coils.

 

[SteveSRT8]

if there's any significant misfire there should be a code.

 

[another Todd]

After moving the coils around multiple times the problem always follows the same coil. This coil was replaced about a year and a half ago, it is a Borg Warner coil. I have ordered a new Delphi (OE) coil from Rock Auto. This car is killing me!

 

[Warstud]

Originally Posted By: another Todd
I used to be able to rev over 5000 rpm.



Originally Posted By: another Todd
This car is killing me!

 

[JTK]

Originally Posted By: another Todd
After moving the coils around multiple times the problem always follows the same coil. This coil was replaced about a year and a half ago, it is a Borg Warner coil. I have ordered a new Delphi (OE) coil from Rock Auto. This car is killing me!


Like I've mentioned on other GM Atlas/Vortec 4.2 threads, ignition coils are a common problem area for this vintage. IIRC, a newer design was used on 2006+ Oil in the plug wells can also cause issues, as does the use of some Bosch plugs.

Joel

 

[another Todd]

I finally replaced the Borg Warner coil that I had installed a year and a half ago. Put in a new Delphi coil. All the problems are gone, and it has been about 10 days now and it runs perfect. Again, the stealer wanted $3,800 to replace the head and it appears now it was just a coil.