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Old 11-19-2013, 02:20 PM   #1
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Default P0300 mystery problem!

Alright folks! I decided to join this forum as I have a wonderful 2000 GM 3100 engine, and I love diagnosing the crap out of these things. Therefor, I don't expect it to take more than a page or two until we find the problem. The problem? pending p0300. First, the car:

2000 Chevy Malibu, 3100 (only engine of that year) Miles: 130,000

Work that was just completed: Head gaskets, milled heads .020

Replaced a few valves due to pushrods incorrectly replaced. After lapping new valves, I stood the heads on their ends and sprayed the combustion chamber with soapy water and used shop air to check how well sealed they are, nearly perfect, very little amount of bubbles (I do this with other head gasket jobs to see if I should re-lap the valves too, just a fun trick I learned from an auto Machinist)

Anyways... intake plenum was cracked too, picked up a used one from pull-a-part. I have a 24" straight edge and check all surfaces since the overheating effed the heads. NOTHING else was warped! Engine block, lower intake top and bottom were fine. plenty of friction with a .001 feeler gauge.

CLEANED and replaced all o-ring bores like the injector bores, coolant bore for the pipe wrapping around to the heater core, the other heater core pipe to the water outlet. It was a very well done job! I like to take pride in surface prep work when I work on my stuff.

So after putting it back together, it had a horrible idle. I check with a scan tool and found fuel trims to be ST: -4 LT: -10 so it'* running rich. I checked for intake leaks, even though that would cause a LEAN condition, but I just wanted to make sure. Forgot to mention, new fuel regulator was put on due to it being leaky in the first place.

Next, I found the O2 sensor to not be able to go over .6v, replaced it and very nice .01v-.92v range. Did not clean up the idle. So strange, it idles rough and will stall with a throttle snap as if it'* lean... but it'* rich!

Doesn't skip a beat when driving, very nice. Just a bad idle. (trying to beat this into your head I guess) Fuel pressure is 50 KOEO, 45 at idle, and 50 with vacuum line disconnected. (Key On Engine Off ..................................... 358-405 kPa (52-59 psi)) <alldata specs

Keeps pressure for ever (I let it stand for 5 minutes) since it didn't drop, ruled a leaky injector out, but then again, that would cause a p030X anyways. pressure is between 45-50 revving up, so fuel pressure not dropping off with higher RPM'*. Intake plenum connected to vacuum gauge, I have 19" of vacuum at 1200 RPM'*, which is when it runs fine. Idles rough at 700-800 rpms, but stopped in gear is worse.

Next thing to note, all other sensors are giving good outputs on the scan tool. Nothing odd or out of range. Took IAC valve out and cleaned it. Both the new and old plenum had gunked EGR ports, so the previous owner didn't have this problem before he blew the head gaskets. In fact, I need to go unbolt the egr and block the port with my finger and see if it'* a hanging up egr....

other quick notes: new ACDELCO plugs and wires from rockauto. New water pump and thermostat. no exhaust leaks up to the muffler (muffler leaks) and vacuum doesn't drop at 2500 RPM'* ruling out a blocked exhaust system. I know, if it runs fine, why eve check? Because I did!

Going out to check the EGR, will come back with results. I CAN'T SPEND TOO MUCH MONEY! :( I'm broke, all parts are going on a credit card. I have an alldata subscription too, so I can get specifications and such (I'm an auto tech, but for Lexus)


Jack A Lane
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Old 11-19-2013, 02:39 PM   #2
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Wow, very well written.

I'm not all that familiar with the 3100, so, I'll start with basics. Since the top end overhaul, have you done a cylinder compression test?

Have you sprayed all around the intake system with carb cleaner while the engine was running?

What'* your scanner saying for a TPS % at idle?
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Old 11-19-2013, 02:40 PM   #3
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BTW, I hate single O2 systems. It makes it difficult to know which bank is doing what.
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Old 11-19-2013, 03:07 PM   #4
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Okay, took EGR off, blocked port with tape. No change. Also noted that there was nothing coming through the EGR valve, so it'* not leaking exhaust into the intake at idle.
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Old 11-19-2013, 03:10 PM   #5
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I have not ran a compression test since then. I only did that to diagnose (really to validate) the blown head gasket.

I did use carb cleaner around every connection you can get to. Also used propane. I'm going to give you a snapshot or two of the parameters. GIve me 10 minutes...
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Old 11-19-2013, 03:17 PM   #6
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Hard to see, but anyways it'* 0.4% after snapping back. tops at 100% (ignore scaling on meter.) Waveform shows no open or shorted spots, though I feel that we both know that the on-board ECM can hide those sometimes, where only a scope will really show that type of fault, if minute. (my-noot)
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Old 11-19-2013, 03:17 PM   #7
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Shoot... I have to host my picture to insert it, don't I?
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Old 11-19-2013, 03:18 PM   #8
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Guess not, just can't upload in quick reply. Here we go!
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Old 11-19-2013, 03:56 PM   #9
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Ok, we can eliminate the TPS causing the PCM to go rich.

Do you have the compression tool? Just to verify that all the cylinders are not leaking somewhere.
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Old 11-19-2013, 04:34 PM   #10
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if the compression is good sounds like a bad injector. you have a scanner to check for individual misfires?
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