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2003 Chevy Tracker 2.5l Idle Misfire/Hard starting when hot

posted May 09, 2016 02:55:37 by Jerry Hardway
I have a 2003 Chevy Tracker with the 2.5L V6 and auto transmission, the guys on the suzuki forum sent me here. Just got it from a buddy who has spent about $2000 trying to fix the problem between the dealership and another garage. Just about every sensor on the engine has been replaced as well as fuel pump and fuel regulator. The only codes it has consistently thrown are lean codes for both banks. Yesterday it finally stored a random misfire code in pending codes.

When it goes closed loop it has short term fuel trims at 19.5% for both banks and long term fuel trims around 15% for both banks. When in closed loop it idles smooth as glass, but as soon as it tries to idle in open loop it starts loping and missing, it sounds almost exactly like a motor with a performance camshaft.

Also, after it gets warm and you shut it off for 10 minutes or so it is very hard to start. I have been probing around with my oscilloscope for the past couple of days and figured out that during the no start condition the computer isn't sending trigger signal to the coil packs.

After some more testing on the Crank and Cam sensors I believe my Cam sensor is faulty causing the no start condition. The cam sensor on this car has two channels, one with 6 pulses per cam revolution and varied pulse width to signal ignition timing and the second with 360 pulses per cam revolution with constant pulse width to give a time reference to the computer.

When I crank the motor over with the injectors unhooked cold the 360 and 6 pulse signals are all there and clean, when it is warm the 6 pulse signal is still good but the 360 pulse signal is very choppy. I checked the voltage and ground to the sensor when it was cranking as well and they were both good.

The strange thing is the cam sensor was just replaced and is still producing the seemingly bad signal. I guess the new sensor could be bad, but I don't want to be too willing to just throw money at it. I've attached snapshots of the scope view during both cases. The 6 pulse signal is in blue and the 360 pulse signal is in red for both pictures.

Just wanted to get another opinion on it and see what you thought.

Thanks.

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33 replies
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RobBrown said May 25, 2016 15:01:24
Have you ruled out a vacuum leak? Stuck open EGR? As Tyler's saying, if there were a fuel control issue (no change in injector pulse width, for instance), your fuel trim numbers wouldn't be improving off idle...if anything they'd be getting worse (or if pulse width was stuck high you'd be smelling a really strong fuel smell from the exhaust and seeing really negative fuel trim number when you switched to closed loop).
Just a dude who found a sudden and unexpected passion to bless folks who can't afford auto repair with free help. I never, ever imagined this is how'd I'd be spending my free time:-)
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RassielCotoMedina said May 25, 2016 19:10:29
the injector PW is not changing cause computer is not listening to o2 at OL it keeps the PW from the closed loop that worked, i would not overanalyze injector PW, smoke test that intake it looks that you have a vacuum leak
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Jerry Hardway said May 27, 2016 18:18:09
Okay, so I decided to take another look at the injector pulse width and I can see a minute change as the fuel trim increases. Very small though...

I decided to do another compression test today and came up with something strange. 5 out of the 6 cylinders had 200-210 psi of compression hot. The sixth cylinder only built 60 psi the first time I tested it. I bled the gauge and made sure it was hooked up right and cranked it over again to get a 0 psi reading. Tried one more time and it jumped up to 170 psi. Two more tries resulted in 180 and 190 psi. I noticed on one other cylinder occasionally it would make one revolution without building much compression. Putting oil in the cylinder built the compression up high enough to peg my compression gauge.

I'm thinking maybe a valve hanging up.

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Tyler said May 28, 2016 14:33:36
I decided to do another compression test today and came up with something strange. 5 out of the 6 cylinders had 200-210 psi of compression hot. The sixth cylinder only built 60 psi the first time I tested it. I bled the gauge and made sure it was hooked up right and cranked it over again to get a 0 psi reading. Tried one more time and it jumped up to 170 psi. Two more tries resulted in 180 and 190 psi. I noticed on one other cylinder occasionally it would make one revolution without building much compression. Putting oil in the cylinder built the compression up high enough to peg my compression gauge.

I'm thinking maybe a valve hanging up.


Strange indeed! The two cylinders occasionally creating low compression definitely suggests a valve sealing problem, but the 200+ PSI compression in the other cylinders is kinda weird, too. I consider that to be abnormally high (usually as a result of late intake cam timing), but that may be normal on this engine.

Unfortunately, I still don't know if that accounts for the lack of fuel control at idle. I was thinking about using a cam/crank waveform, but then I remembered how this engine only has one sensor for both.

Any luck finding a vacuum leak, or an EGR problem?
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Jerry Hardway said May 28, 2016 19:28:50
The spec from the manual for this motor is 200-230 psi so it isnt high in this case. I found one small vacuum leak but it didn't help matters any. EGR was supposed to have been cleaned and seemed to function properly when I removed it.

I decided that since the motor had a light knock seeming to come from the bottom end and the weird compression I would start to pull it apart and check for valve timing and other issues. I found that the passenger side cams were put in one tooth out of time, probably during a timing chain replacement as this motor has 155,000 miles and still had a very tight timing chain. Going to have the heads rebuild and fix whatever is causing the knock as well.

Figured it is better to make sure there are absolutely no mechanical issues and then go from there.
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Tyler said May 28, 2016 21:14:42
The spec from the manual for this motor is 200-230 psi so it isnt high in this case.


Wow, 230 PSI? That's a hard working engine!

I decided that since the motor had a light knock seeming to come from the bottom end and the weird compression I would start to pull it apart and check for valve timing and other issues. I found that the passenger side cams were put in one tooth out of time, probably during a timing chain replacement as this motor has 155,000 miles and still had a very tight timing chain. Going to have the heads rebuild and fix whatever is causing the knock as well.


Ruh roh :-( I like your address-the-mechanical-issues-first philosophy, and it'll give you an opportunity to perform any other needed repairs at the same time.

I am curious to hear how the fuel trims change after the chains/valvetrain get fixed. Typically, I expect a timing issue on a OHC V6 to cause a trim imbalance from one bank to the other, but this one may be different.

Sorry to hear that it came to this, sir. Thanks for keeping us informed on your diagnosis, let us know what else you find!
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JerryHardway said Jun 18, 2016 21:34:00
Okay, we are back at the diagnostic game. Got the rebuilt engine dropped in and fired up and its doing the same old thing.

Just as a reminder and an update on the symptoms here is what i'm getting: rough, loping idle, lean codes both banks, random misfire code(from lean condition I assume), most of the components have been replaced(new injectors, new coils, new TPS, new MAF, new IAC, new cam sensor, new crank sensor, new upstream O2s, new fuel pump, new fuel pressure regulator, new temperature sensor, new intake air temp, new alternator, and probably some new stuff I don't know about), freshly rebuilt motor, compression 200+ psi on all cylinders now, fuel pressure 35 psi at an idle, both upstream O2 sensors read flat lean on the scan tool when at idle or when revved up now(haven't tested it on the road yet), STFT 19.5% when it decides to go into closed loop(LTFT was going to 15% before but it won't change for a while since I had the battery unhooked), MAF reads 1.85 volts at an idle, TPS adjusted properly, ignition timing set, EGR valve works freely and the coil resistance is correct, it runs some better and quits loping if you unhook the fuel pressure regulator or put propane in the intake but is still dead lean.

So, did some more testing and came up with some rather unusual results. I decided that if it had a large enough vacuum leak somewhere to cause my problem that I could block off the IAC and it would still run(and maybe run better). Well, I did that and it wont idle with the throttle completely closed.

What seems strange to me is the what happens when I get it to idle at about 400-500 rpm with my foot. It will idle smooth as glass right here, it will stay in closed loop, the O2 sensors switch nicely and the fuel trims will drop down to 5-10%. If I increase the idle speed to just 600 rpm it will start loping, the fuel trims will peg, the O2 sensors go full lean, and it won't stay in closed loop for very long. Also, it stays lean throughout the rpm range with the IAC blocked off.

Any ideas?
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Johan said Jun 19, 2016 08:59:47
Have you measured the inputs at computer connection could be a wiring problem at for example TPS.
Swed
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JerryHardway said Jun 19, 2016 18:26:58
Yes, i've been measuring everything back-probed into the computer connector.
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AndresRohena said Jun 20, 2016 02:13:34
Did you check the computer itself? Check for those outputs and inputs I read of this post and you have done every test,check thecomputer measuring 5volts supplies for the all the sensors! Keep us updated!
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JerryHardway said Jun 20, 2016 02:32:19
Okay, I will make sure the computer is putting out the correct voltage when I get back on the weekend. I pulled the computer out a few weeks ago to do a visual inspection on it and I couldn't see and swelled caps or anything that looked suspicious. I will be away this week but will report back my findings when I get home and get to work with it some more. If anybody else has suggestions feel free to chime in.
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RobBrown said Jun 21, 2016 18:13:18
Quite a battle, this one! If I just read the symptoms as presented in your latest big update (post-rebuild-installation), I hear fuel delivery issue or MAF issue. As I reread this post I realize that you've had some "in spec" readings on both fuel pressure and MAF at idle, but I don't see any indication from you of those readings at different engine loads.

I'd really like to see fuel pressure readings at idle, with regulator unhooked at idle, at 3000 RPM, and a description of FP behavior on a snap throttle. As well, I'd like to know what your fuel trims do now at wide open throttle during a road test...it sounds like they initially were going rich during your initial testing...but it seems like the behavior may have changed since the rebuilt engine was installed? The fact that it runs better (or seems to) with the fuel pressure regulator unhooked from vacuum also seems to be a significant clue. That's increasing pressure in the rail, and in so doing makes me wonder about the fuel delivery system.

Also, I'd really like to see the MAF readings from idle up to 2500-3000RPM.

You've been through a lot with this beast, but maybe best not to assume ANYTHING given the engine has been recently rebuilt. Kind of like starting from scratch:-(

Best of luck to you!
[Last edited Jun 21, 2016 18:26:01]
Just a dude who found a sudden and unexpected passion to bless folks who can't afford auto repair with free help. I never, ever imagined this is how'd I'd be spending my free time:-)
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JerryHardway said Jun 21, 2016 18:27:25
The fuel pressure with the regulator unhooked at idle is 44 psi. And at 3000 RPM(free revving) it is still around 35 psi. With a throttle snap it will jump up over 40 psi then drop below 35 and then come back up to 35 psi(in sync with the vacuum readings).

The MAF at 2500 RPM(free revving) says around 11 g/s on the scan tool, i've never checked the voltage output there.

As for wide open throttle it always stays in open loop, but it will go rich at that point.

The only time it would stay in closed loop is when I blocked off the IAC and forced it to idle at 500 RPM(lower than normal).
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JerryHardway said Jun 30, 2016 03:10:32
Okay, so broke some very serious ground tonight. I revamped my smoke machine to produce smoke a little better as well as changed my smoking method and I believe I have located the root of my problem. I first found a small leak around the MAF and fixed it which seemed to make it run some better. Further smoking revealed a rather large vacuum leak coming from the EGR actuator shaft which i confirmed by spraying brake cleaner around the shaft with it running. I pulled it off, hooked my vacuum pump up to the intake side of the valve and it wouldn't hold vacuum no matter how fast I pumped.

I tore it apart and found a pressed in graphite like sleeve that the valve itself rides in which I assume acts as a seal as well because there was no conventional seal around the shaft. I would like to fix this one vs buying a new unit because the seal around the shaft is the only problem with it.

Does anybody have any idea what kind of sleeve is around the stem of the valve and any way I could replace it or something I could use to replace it?
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JerryHardway said Jul 03, 2016 19:07:03
More bad news, I decided to block off the old EGR and see if it helped my problem and although it was better it still runs severely lean. I smoke tested again and I'm fairly confident I have no vacuum or exhaust leaks.

Any ideas?
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