P0121 code and the symptoms I'm seeing - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 04-15-2008, 01:57 PM   #1
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Default P0121 code and the symptoms I'm seeing

OK, I'm suspecting a bad repair job by the local dealer. My 2000 SSEi went in for the lower intake manifold gasket repair. I get the car back and for the first few miles, it seems fine. Then I get a check engine light. I notice the idle is up a bit and the vacuum/boost gauge is not reacting to engine conditions like it used to. I travel to my destination and notice at idle, out of gear, the idle is 1100 rpm and the boost gauge is reading 0. The last several miles, it never went below 0, and most of the time it was at 4 psi doing 65 and only 1800 rpm. Definitely not right.

I go get my scan tool, check for codes, P0121 TPS Sensor. I can understand a bad TPS causing a high idle condition, but the boost gauge is my big puzzle here. Get it home, time for bed.

Next morning, I hook up the scan tool and verify the TPS, ignition ON, engine OFF, I run the throttle to WOT. Smoothly and slowly, the TPS readings in the scan tool read from 0 to 100% in smooth ascending numbers...this doesn't appear to be my problem. I drive the 20 miles to work paying close attention to the car. Every time I come to a stop light, I notice I come to a full stop and the idle is at 980-ish, then slowly falls to 650-ish (about 5 to 8 seconds time to do this). Boost still reads -4, never any lower.

When I hit the couple miles of interstate, giving throttle is odd. Car hesitates and then BAM!, like Emeril, it drops into low and the RPM'* skyrocket to 4000. Couldn't see scan tool for TPS reading at that time as I had traffic to deal with. I continue to work. In the parking lot at work, I pull in and park, and check final readings. TPS still reads 0 with no throttle, idle slowly lowers again like before. Odd part is the boost gauge reads about 4 or 5 PSI parked. This is wrong!

I shut off the car and go back to ignition ON Engine OFF again to verify the TPS again. Same as before, smooth from 0 to 100%, no odd variations. Boost gauge still reading 4 PSI. MAP reads 28.2" with the car off. This isn't right either. Turn key off and gauge drops, turn key on and gauge comes back up to 4 PSI.

I'm now waiting till I get to leave for the day and see what I get when I go out to the car. It seems I have a problem in the manifold pressure somewhere, but I don't know what would cause it.

Any ideas? I have an appointment to take it back in to be looked at Thursday night, but if I can point to the problem, it would be helpful. I'd like to be able to say "It appears to be 'this', and that is a direct result of the LIM repair, please fix it". I don't want to begin playing the "let'* throw parts at it" game. Plus the TPS seems to be functioning normally from what the scan tool shows me.
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Old 04-15-2008, 04:16 PM   #2
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can you verify all the vacuum fittings or take a picture of them so we can look them over?
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Old 04-15-2008, 11:28 PM   #3
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I agree that it sounds like a vacuum problem. Check the T from the LIM under the supercharger snout carefully.
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Old 04-15-2008, 11:40 PM   #4
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That really sounds like a vac leak. Get some starting fluid and spray around the connections. Dont forget to get around your EGR tubes. If RPM goes up, that'* it.

Vac leaks will give strange readings. I had a leak on my EGR and had codes for lean, rich, and pressure control solinoid, as well as missfire. All went away when the leak was fixed.
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Old 04-16-2008, 02:31 PM   #5
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Thanks for the recommendations. I was suspecting a MAP problem, and you guys have lead me in the right direction. OK, here'* what I found (once I had daylight to look)....

A vacuum line was off what looks like a FP reg to me...I haven't studied this motor enough to know all the parts yet. So I put it back on.


I also found a very poor connection at the TB. This line appears to have a molded fitting, and it'* been taped over to attempt to repair it. Talk about a stupid repair.


I don't know if the dealer did this, or the previous owner, but it'* gotta get fixed. I know the dealer wasn't the sharpest knife in the drawer as they left my oil pressure sender disconnected when I took it in...so I suspect they weren't that thorough putting things back together.

I followed the lines as best I could. The T fitting under the snout looks pretty new. Maybe they replaced it. I find no other lines off.

Driving test and scan tool results. Cleared the code and drove. Vac is showing -9.5 again like it should (8.5 to 9" on the scan tool for MAP). This is good. Throttle did not make any weird swings like the other day. Tested TPS again, full range of movement results in full 0-100% swing on the scan tool. Still good.

My only question is the idle. It still holds at about 850-900 rpm when I come to a stop, and slowly lowers to about 650-ish when remaining stopped. This still takes about 5 seconds. Is this normal behavior? I never really paid attention to it before. It'* not a killer, but if it indicates I still have a problem, I'd like to find it. I also tried a test while running about 40, I dropped into neutral, and the idle stayed at about 1400 rpms. I would have expected it to fall to 700 or something near there. But then I'm used to the old turbo Buicks, none of this new stuff...so this is a learning curve as to what is normal and what isn't.
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Old 04-17-2008, 12:26 PM   #6
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Day 2, all'* fine with the car running. No more problems experienced. Tonight I return to the dealer to find out about the taped over vacuum line and ask for a new one. Anyone know the part#, or is this just a piece of hose used to join the hard line to the TB fitting?
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Old 04-17-2008, 03:27 PM   #7
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The one you have pictured looks like just a length of tube, but it is actually supposed to be a molded 90* angle. The dealer will probably be able to get you one, but you will have to buy the entire vacuum line kit ($20-30), which is NOT a bad idea!
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Old 04-18-2008, 01:38 PM   #8
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Looks like I'll be on my own doing just that. Dropped by the dealer last night, and the service manager and a technician looked at the fitting (as I voiced my displeasure at having to finish fixing my own car after they got hold of it). Get this, the technician said "well, if the vacuum is holding, then that'* OK like that". Give me a break!

Thanks to all the BC board members that care more about my car than the worthless techs I've had to deal with recently...this is one good bunch of folks. THANKS!
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Old 04-18-2008, 01:57 PM   #9
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did your fitting look like that before you brought it to them? did they make that electrical tape fitting? If they are happy with that you need to find a new dealer. That is ridiculous.

I'm guessing they broke it upon removal.
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Old 04-21-2008, 01:51 AM   #10
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Well judging by the pictures the first line left off was the fuel pressure regulator and the second was the main vacuum conection fitting that very commonly gets brittle from the high engine compartment temps. that the supercharged engines produce. I am a pontiac technician and I know for a fact them lines break very easily. I break them regularly and swallow my pride and order the customer a new vacuum harness. I can see it is time to make sure that person NEVER works on your vehicles again. Saying that electrical tape a good enough repair for a vacuum leak is like saying duct tape can fix your tires. :P People make mistakes, it'* human nature, but to be so inconsiderate as to lie to you like that is unacceptable. Gives the good mechanics a bad name.
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