DTC p0304: misfire, cyl. no. 4, '99 SSEi - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 01-04-2013, 01:54 AM   #1
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Default DTC p0304: misfire, cyl. no. 4, '99 SSEi

Hi to everyone, a newbie here, with request for help or suggestions in resolving a "miss" which progressively worsened. I first noticed a slight miss in the engine on acceleration, lead-footing it, back maybe a couple of months ago. The car is my wife'* DD, with about a 16 mile commute one-way, mostly over 2 lane highway, with some 4-lane and about 4-5 miles of in-town. The way my wife drives, she did not notice a problem until the car would only do about 45-50 mph up the hills on a 4-lane on the way to work. The CE light did not come on for quite a while after i began noticing the miss, but finally did. After seeing the code, I pulled the plugs. Cyl. 4 plug had a noticeable crack in the insulator, nearly the whole length of the insulator. Another plug (rear, driver side) had a smaller crack. I replaced the plugs, (Delco, stock spec.) which only had about 45K miles on them. The set that had the cracked insulators was installed at about 121,000 mi. The car currently has about 166,00. Did not replace all plug wires, which have slightly fewer miles. I did replace the wire on middle front cyl, in which the connector had come loose on pulling the wire. The car seemed to have more power on acceleration, though the misfire did start occuring again after a couple of times of hard acceleration. I replaced the fuel filter, with no improvement, though it was time for the filter to be replaced. I added some injector/intake cleaner to the tank, and so far have run about 1/2 of that tank through. I have occasionally put some top-end/injector cleaner in the car, especially in the last 25K or so miles, but thought maybe the misfire could be a clogged injector. Have had no change in misfiring since adding the fuel treatment. Then I replaced the front coil, again with no change in the misfire. It does seem to run better when cold, and to misfire when it gets warmed up, and with more aggressive throttle app. My wife drove the car today to O'Reillys after I called and asked if they would again put their code reader on it. She said she didn't notice any missing of the engine or rough running, though the CEL is on and the code was there today. How she drives, however, is very gently (which is fine), and way too slow for me.

Print-out from O'Reillys code reader says two reported fixes are a) replaced valve spring(*) and b) replaced PCV valve Grommet, with a p0300 code.

Does anyone have an idea of how likely it is that this may actually due to a broken valve spring, or any other suggestions? Could one of the other coils somehow be causing a code for cyl. no 4? I plan to put the one new coil I bought in the other two positions to see if that has an effect on the misfire, as well as try a new plug wire on cyl. 4, though have not been able to do these tests yet.

Thanks for any help with this, Joe
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Old 01-04-2013, 08:36 AM   #2
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Joe,

welcome to the forum. Not sure about the valve spring concerns, but the misfire problem under load, especially uphill has been a common problem for many of us.

If I was in your shoes, I would replace all the wires, and also look for corrosion on the coils where they make contact with the plug wires. Generally, the misfire under load races to a bad plug, plug wire, coil, or coil corrosion contact (at the plug wire).

Since you have replaced the plugs, you might just replace the wires, they need to replaced anyways- and see where you are at.
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Old 01-04-2013, 08:40 AM   #3
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is it 304 or 300, possible causes are much different.

the wire set you bought doesnt sound good. most times its fuel related if its high rpm, filter, pump or fpr, causing it to be low pressure. take the vac line off the fpr and sniff it, if it smells like gas its bad. if you have a gauge check the pressure and pull the line see if it goes up about 10
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Old 01-04-2013, 09:28 AM   #4
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I agree with Justin. You are approaching this as though the code is P0304, but when you had it scanned the code was P0300. You need to know which it is because the fix for each is very different.

Start with replacing all the wires, not just a few.

Get the fuel pressure tested.

Place one end of a wooden dowel against each injector and listen to see if you can hear if firing in a regular pattern. A mechanics stethoscope (~ $10) would be better.
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Old 01-05-2013, 01:27 AM   #5
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Michael, thanks for the pointers. It is interesting to hear this is a common problem. This car has been problematic almost since new, worst problem being the body control module setting off a "brake lamp" notice on the screen, since the car was about 4 months young, that GM/Pontiac and the dealers just would not fix. The interior is of poor quality - I have repaired and replace plastic panels behind door handles, trim has fallen off, shock setting switches both indicate they are "on", etc., but I have always told my wife the drive train seems to be very sound. Never a problem with it running well until now, with exception of slight problem at 12x,xxx miles, which cleared when I replaced plugs and wires. So now the wires have about 40-45K on them, with Bosch wires which have a 100K warranty. It could be, and hopefully is, a plug wire on no. 4 cyl., so tomorrow I will replace all the wires. Will also check and probably replace the PCV valve grommet, which is one of the reported fixes on the O'Reillys printout. Today a guy suggested checking the compression on cyl. 4 and the one opposite it (I am still not familiar with all the cyl. no. positions) after I asked him what he thought about the idea of the miss possibly being a broken valve spring. So I will borrow a compression tester and use it on two cyl'*. Not having used a compression tester on an engine without a distributor, I am just wondering out loud if there is an easier way to disable the ignition while doing the compression test, than pulling all six plug wires off the coils, Not that it is that hard to pull the wires from coils, will just have to mark them with a marker.

Justin and Roy, the O'Ry'* printout specifically states p0304 as the trouble code at the top of the report. Under Code Summary and Reported Fixes it has two items. The first is the valve spring replacement, second fix says "Replaced PCV valave grommet w/p0300", so that is where the p0300 came from. The code given on my car is not p0300, but at 166,000 miles m/l I will check the grommet. Another problem the car does have is quite a bit of oil leaking which appears to be from the rear rocker cover gasket, as well as the front, but the rear looks to be worse. Maybe some of that is from the PcV valve grommet?

If the wires and grommet don't fix it, will use a stethoscope on injectors, and hook up a fuel press gauge temporarily.
What should the pressure run at idle? I think that Justin is referring to pulling a vacuum line, but I am not sure which line and where the line you might be referring to is. I am used to pulling the line off the FPR on my 5.0 Mustang to check/set pressure, but I don't know about these GM 3.8 engines. I guess I will just follow the fuel rails until I see the FPR and look for a vac line.

Thanks again guys, will report back after these fix attempts.
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Old 01-05-2013, 06:25 AM   #6
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The first thing I would recommend if you can get one is a at least a OBD pocket scanner. They run for about $50-60. Why do I tell you this? It will save you the headache of running back and forth all day long after you swap parts around to see if the code is still there. And with a P0304 and a P0300, you could be checking for those codes all day long. And with some codes, you may have to wait a few drive cycles for the test to pass before it will or not post a code.

Anyway, the P0304 can sometimes be pretty easy on a 3800. In this case, its cylinder #4 misfire. It can either be...
Sparkplug
Sparkplug wire
High voltage module
Or injector.
Now this is where having your own scanner would come in handy, because, instead of buying NEW parts, we can swap sparkplugs from #5 to #4, and test for misfire. Then you can do it again with the sparkplug wire, and then the high voltage module. And then the worst case, swap injectors around. Remember, if the engine has 6 identical parts, why not swap them to see if the problem will follow it??

P0300, misfire on all cylinders. If this is accurate, it could mean you have a vacuum leak somewhere between the throttle body and the cylinders. But, lets get the P0304 fixed first.
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Old 01-05-2013, 10:09 AM   #7
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fpr was for 300 effecting all cyl, but if its 304 do as mike says. they are in order of most likely. ive never heard of a 3800 with bad springs or pcv gasket. those are general sugestions for all engines and shouldnt be regarded. as far as the leaks gaskets are cheap its just a pain to get to on the back, but its not rocket science just take stuff off till you can get to the bolts
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Old 01-07-2013, 01:33 AM   #8
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Thanks again guys for the good suggestions.

First, the problem appears to be solved. Apparently replacement of plug wire on cyl. 4 did the trick, although I replaced all the plug wires yesterday with exception of cyl. 3, which I had already replaced a few weeks ago because the connector came off the wire when I was changing plugs (again, the plugs and wires all had no more than about 45K miles on them.) At that same time I replaced what I call the "coil" for cyl. 4 &1, which is what I assume Mike is more properly calling the "high voltage module". That was my unscientific "throwing parts at it" approach. Though I know testing saves $$ compared to the guessing game of choosing a part to replace, based on experience with our '89 SSE I really thought it was a hi-volt module. When I saw the cracked insulator on two plugs, I thought that was it, but it wasn't. It would have been much smarter, and cheaper, to trade plug wires or better, to just put a good one on cyl. 4, the way it ran better when cold then gradually started missing as it got hot really made me think it was like an old style coil going out.

The no. 4 plug wire i removed didn't look bad at casual glance, and by that time I just wanted to get the job done so didn't look at it closely. However, the cyl. no. 1 plug wire had a place that was rubbing on the fan guard, and was worn through about half way, definitely through the stainless steel winding. The OReilly code reader printout, to repeat what I said above, stated the fault code was 0P304, a misfire of cyl. no. 4. So I take it that there was a problem with the wire on cyl. 4, since I had replaced the plug and coil/hi-volt for cyl. 4 and the misfire was still present, and after replacing all the plug wires it runs great. Again, there was definitely not a 0P300 code in the PCM, that was just from a reported fix for a 0P300 on the printout, I suppose they include that becaused it is another source of a related problem (the OReilly printout did not explain what a 0P300 code means, and I don't know the definition of it).

We plan to sell the car when my wife retires a year and a half from now, so hopefully I won't be dealing with the issue again. However, if I for some reason I decide to keep the car much longer I might buy a good scanner to save some time, as Mike suggested.

For my extra expense from my "try this part" diagnosis method, at least I have an old "coil" unit that seems to be good, and an extra new plug wire that I could use for testing if the problem occurs again.

The other current problem on the car, besides the oil leaks, is that the black plastic box on the firewall, that I would call the HVAC core cover and fan unit, is crumbling apart, from effects of heat and age it appears. I plan to use some JB Weld and black silicon to patch and seal it with. I would like to fix the oil leaks, at least the rocker cover gaskets, but since I have five other vehicles I am messing with, not counting the one I really would like to work on (my 5.0 Mustang has a dead engine), along with all other things in life, I probably won't get around to replacing those rocker cover gaskets - maybe the front one.
For now, the wife'* DD is running better than I think it has in several years, so she is happy, one of life'* simple pleasures. Thanks for the help guys.

I'll be back, and will try to post my pics of our white '89 SSE parked next to Boyd Coddington'* 57 Chevy, "Chezoom", at the '93 Rod & Custom Ameri-tour.

Cheers, Joe (aka, Charles)
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