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Old 04-12-2009, 11:56 AM   #1
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96 LT1 P0300 Random Misfire - Low Fuel Pressure?

My 96 LT1 has about 54,000 miles. It is getting a P0300 DTC. It idles rough, is missing, and makes no top end power. It is mostly stock and has always run good. I parked it in the garage one day and it was running fine. When I got it out like two weeks later it was running bad. It was real low on gas when I parked it.

I checked the fuel pressure thinking maybe I fried the pump. Here is my readings:

43 psi key on and primed
36-37 psi at idle
When revving, it jumps to 42 psi drops to 35 psi and then back to 37 psi at idle
46 psi with vacuum line disconnected from FPR. Pressure stays steady when revving. The car seems to idle smoother and rev better with the vacuum line disconnected.
Pressure drops to 34 psi when I turn the engine off. It seems like it will hold pressure for like and hour or two. I checked it after 24 hours and it was 0.
I have not tested the pressure while driving yet.
It seems like the fuel pump kind of groans when it primes. It doesn't really sound like it used to.

Plugs and O2 sensors are about 2 years old. Fuel filter is new. Pretty much everything else is original.

How do my pressure readings look? Any other suggestions to help me get my car running good again would be appreciated. Thanks.
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Last edited by Casey96SS; 06-11-2009 at 10:03 PM.
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Old 04-12-2009, 12:11 PM   #2
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Gauging your fuel pressure test results, I would rule out the fuel system. Those numbers look fine.

May want to look into diagnosing your optispark/coil/ICM/wires.

Perhaps something chewed through a plug wire while it sat for 2 weeks. Look under the hood at night and look for jumping spark.
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Old 04-12-2009, 09:32 PM   #3
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That thought actually crossed my mind about a mouse chewing up some wires or something. I'll start checking into those things. The only reason I thought it might have been the fuel pump was because I let it get so low on gas.
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Old 04-12-2009, 11:27 PM   #4
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Test it under full load at max RPM. Pressure should be close to the "no vacuum" reading. Anything below 40psi at WOT/max RPM is a problem.
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Old 04-17-2009, 06:14 PM   #5
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Just got back from doing a test drive with the fp gauge taped to the windshield. I can't imagine how stupid that has to look from the outside. It was pretty difficult to read the fp, try not to hit anything, and watch for cops all at the same time.

Anyways. Part throttle driving, fp was about 35 psi. Under WOT, the fp would jump to around 42. At redline, the fp would taper off to about 39 - 40 psi. It would usually drop to about 38-39 right before the rev limiter kicked in.

Driving the car, it seems to run fairly rough at low RPM. It tends to chug when you engage the clutch. At high RPM, it does not have a noticable miss, but it does not seem to be making the power it used to. It runs fairly steady at constant part throttle.

What do you guys think about the fuel pressure? Is it border line?

My next tests are going to be to test the plug wires with a spray bottle of water at night. I am also going to test the resistance of the coil to see what that looks like. If the ICM was bad, would the car not run at all? The car runs the same whether it is hot or cold.
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Old 04-18-2009, 08:55 PM   #6
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Tested my plug wires out at night. No arcing that I could see.

Any opinions on my fuel pressure while driving? Thanks.
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Old 04-18-2009, 09:05 PM   #7
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Fuel pressure sounds normal. The fuel pressure gauge at any time will be the "no vacuum" pressure, minus the intake manifod vacuum. That's why its 35psi at cruise (high vacuum), and jumps to 42psi when you open the throttle..... manifold vacuum is reduced when you open the throttle. At WOT, the intake vaccum is representative of the pressure being lost in the intake ducting. There's always some resdiual pressure loss, even with a reasonably efficient intake system.
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Old 04-24-2009, 01:41 PM   #8
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I did some more testing on my car. First I tested the voltage at the ICM connector.
Green (D): 11.88 VDC key on
Pink (A): 11.88 VDC key on
White (B): 1.8 VAC while cranking
Black (C): Grounded

Looks like that is good.

Checked the coil resistance. 5,660 ohms on both terminals. Looks like that is good.

Any suggestions on what I should try next? Seems like I have covered most of the easy stuff. I hate to spend the time replacing the plugs and wires or opti and not have it fix the problem.
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Old 04-25-2009, 12:20 PM   #9
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I,m having the same sort of issue. Just freshend up the engine after previous owner chewed up the cam. Car did this before engine overhaul and now still does it. New Plugs, new Injectors no help. You figure something out PM me or I,ll check back here. A PM would be nice though. If I find something I,ll Let YOU know too.
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Old 06-11-2009, 10:16 PM   #10
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I have arranged for an AutoTap scan of my car tomorrow.

I was checking some things on my car tonight. I noticed when I turn the key on, the fuel pump takes about 30 seconds to prime. The pump also sounds quiet and slow when it is priming. I have always let the pump prime before starting my car. It used to take about 2 or 3 seconds to prime and the sound was quite noticeable. Yes, the battery is chargerd.

It seemed like my pressure readings were okay, but the pump defineately does not sound right when it is priming. Any second opinions that the pump might be bad?

Also, anything particular I should look for when doing the AutoTap scan tomorrow?

Thanks
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Old 06-12-2009, 03:05 PM   #11
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Cant rule out your OPTI or Plugs. I changed my plugs ( guess we didnt change them after rebuild, my bad.) Runs alot better but now its more RANDOM misfires than before.
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Old 06-13-2009, 09:13 AM   #12
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Well I got it figured out. Burned #2 plug wire. AutoTap was showing a high amount of misfires on cyl #2. I was also getting a few misfires on #1 and #4. Those turned out to be false misfires. I started checking out #2. As I was investigating how to get the #2 plug out, I just barely caught a glimpse of the #2 plug wire. I was going staight up off the plug and was wedged right against the manifold. I have no idea how this would have happed. I have the stock plug wires with manifolds. The #2 plug wire would have had to go against gravity, rotate 180 deg, and wedge itself against the manifold. All I had to do was rotate the plug wire back around away from the manifold and it runs great now. I will probably see if I can drive it as is for the summer and then put a set of wires over the winter.

I took it for a ride afterwords with the AutoTap. Zero misfires on any cylinders. I am however getting a P0332 code which is related to the knock sensor. I did seem like I was getting a high amount of knock retard. I would get about 3 or 4 deg every time I pressed the gas. Under full throttle over 5,000 RPM I was getting like 10 to 12 deg KR. I will do some searching on this and post another thread if need be.

I owe a big thanks to Joe from AutoTap. He was very helpful and good guy too. I was really impressed with how the software works. I definately recommend AutoTap for anyone considering scanning software. Scanning and data logging is the only way to go when you are trying to solve problems like this. This problem was a PITA to figure out, but I definately know a lot more about my car now.
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Old 06-13-2009, 12:18 PM   #13
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Quote:
Originally Posted by Casey96SS View Post
Well I got it figured out. Burned #2 plug wire. AutoTap was showing a high amount of misfires on cyl #2. I was also getting a few misfires on #1 and #4. Those turned out to be false misfires. I started checking out #2. As I was investigating how to get the #2 plug out, I just barely caught a glimpse of the #2 plug wire. I was going staight up off the plug and was wedged right against the manifold. I have no idea how this would have happed. I have the stock plug wires with manifolds. The #2 plug wire would have had to go against gravity, rotate 180 deg, and wedge itself against the manifold. All I had to do was rotate the plug wire back around away from the manifold and it runs great now. I will probably see if I can drive it as is for the summer and then put a set of wires over the winter.

I took it for a ride afterwords with the AutoTap. Zero misfires on any cylinders. I am however getting a P0332 code which is related to the knock sensor. I did seem like I was getting a high amount of knock retard. I would get about 3 or 4 deg every time I pressed the gas. Under full throttle over 5,000 RPM I was getting like 10 to 12 deg KR. I will do some searching on this and post another thread if need be.

I owe a big thanks to Joe from AutoTap. He was very helpful and good guy too. I was really impressed with how the software works. I definately recommend AutoTap for anyone considering scanning software. Scanning and data logging is the only way to go when you are trying to solve problems like this. This problem was a PITA to figure out, but I definately know a lot more about my car now.
If you hook a scanner or use a heat gun on mine it tells me #2 as well, however Ive changed my plugs and wires and still get the random misfireing. Its not as bad as it was before but its still driving me crazy.
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Old 06-13-2009, 01:23 PM   #14
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Quote:
Originally Posted by Casey96SS View Post
......
I took it for a ride afterwords with the AutoTap. Zero misfires on any cylinders. I am however getting a P0332 code which is related to the knock sensor. I did seem like I was getting a high amount of knock retard. I would get about 3 or 4 deg every time I pressed the gas. Under full throttle over 5,000 RPM I was getting like 10 to 12 deg KR. I will do some searching on this and post another thread if need be.....
P0332 indicates a problem with the voltage on the knock sensor circuit. When this code sets, the PCM defaults to a calculated knock retard, based on feedback from other sensors (e.g. - MAP, to indicate engine load). You will have knock retard virtually all the time when P0332 sets.

Make sure the wire is still on the knock sensor. If it is, pull the harness connector off, and measure the voltage between the pin in the harness connector, and the block/ground. Should be 5V. If you have 5V, with the harness still disconnected, measure the resistance between the pin on the knock sensor and the block. Should be about 100Kohms.
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Old 06-13-2009, 01:23 PM
 
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