Weird DTC's from a misbehaving '98 Bonnie SSE - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 04-27-2004, 02:32 AM   #1
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Default Weird DTC'* from a misbehaving '98 Bonnie SSE

Help? We have a 1998 SSE with 60K on the clock. The car is impeccably maintained by me and completely stock except for two things. The only mods are a full core, high efficiency radiator and a retrofit of a late 80'*/early 90'* Cadillac FWD trans cooler (identical to the tranny cooler used on the 1992-1993 SSEI'* - BTW, this is a very easy, bolt-in operation with little to no mechanical skill required).

Here'* the problem: the car throws intermittent DTC 301, 304, and 401 without any predicatble pattern. The car is driven mostly around town with occasional short runs at freeway speeds of 75mph. Some of the recent service items in no particular order are: plugs, wires, PCV, fuel filter, air filter. The car is lubricated with Mobil 1 synthetic with filter changes at 2,500 miles and oil/filter changes at 5,000 miles. Transmission fluid is dumped (via a welded pan drainplug - oops, I forgot about that, I guess that is mod #3) at every engine oil change; trans filter is done every 10K. The ignition control module and all coils have been swapped with not one, but two different sets of known good units. Plugs have been changed twice and wires randomly swapped with other cylinders. The voltages on both O2 sensors are within spec and the engine coolant (new Dexcool 60% with distilled water) runs at 199 degrees at full operating temperature. Fuel pressure is right in spec. EGR has been replaced twice; once by me and once by the local Pontiac service dept. Both EGR units have been removed and tested on my 98 SLE and test fine. The aforementioned Pontiac Service department hasn't got a clue: the technician was pointing to cylinder #5 and was insisting it was cylinder #4. I am beginning to think the PCM may be at fault, but, of course, the Pontiac people aren't agreeing.

Looking for any insight into this condition of poor drivability and sporadic DTC appearance.
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Old 04-27-2004, 02:41 PM   #2
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Have you looked at the full descriptions on those codes from the dealer service manual?
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Old 04-27-2004, 11:40 PM   #3
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Unfortunately, the shop manual has not been very enlightening on this unique situation. Each of the DTC'* has occurrs individually without rhyme or reason, so to speak. The code will set but will not immediately reappear after it is cleared (i.e., does not immediately duplicate). Each flow chart for the individual code has been explored, at least as far as the non-dealership mechanic (me) can go without the dealership'* more advanced electronic diagnostic equiment.

Part of the motivation behind my posting was the hope that someone else out there might have encountered the same or similar set of circumstances.
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Old 04-28-2004, 08:20 PM   #4
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HI i have a 97 bonneville that drops a P1406 [egr valve code]....changed the egr with a gm valve...ran smooth for about 2 months...now the code is back again...i disconnected the battery overnight and erased the code....this fix lasted for a day....the code has returned....also unplugged the egr valve....the idle smooths out....am beginning to think it is the ecm....
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Old 04-28-2004, 10:49 PM   #5
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According to a manual, thats a cyl #1 misfire, and a cyl#5 misfire, which means diffrent coils. Its not likely for two coils to go south at the same time. I had this problem once but only with one cylinder and it wsa right after a plug change. I uasually check gaps as i install but one plug was opened up way too much. how it happened I dont know, but it did. I went through everything before i questioned those new plugs. Anyway, that cleared it. Funny thing is, I never felt it Miss. It may be worth looking at those new plugs...
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Old 04-28-2004, 10:51 PM   #6
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Quote:
Originally Posted by SMALLBLOCK
According to a manual, thats a cyl #1 misfire, and a cyl#5 misfire, which means diffrent coils. Its not likely for two coils to go south at the same time. I had this problem once but only with one cylinder and it wsa right after a plug change. I uasually check gaps as i install but one plug was opened up way too much. how it happened I dont know, but it did. I went through everything before i questioned those new plugs. Anyway, that cleared it. Funny thing is, I never felt it Miss. It may be worth looking at those new plugs...
I had to coils go at the same time and it was a pain in the a$$ because we just thought it was one and had to trace the other as well.
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Old 04-28-2004, 11:20 PM   #7
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Your right, coils could open up at the same time. For that matter, I suppose all three could go on strike given the right enviroment. It just doesnt seem likely in sle 98'* case. For one , if #1 cylynder was missing due to a faulty coil, than the companion cyl. would likely miss also. Cyl. # 5 is also Missing according to the code '305'. I if this was due to it'* coil, it seems likely cyl#2, it'* companion cyl. would miss. Thats why I would look at those plugs, wire, and those troublsome boot sheilds that dont do much in my opinion but leak voltage off to ground.
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Old 04-29-2004, 01:17 AM   #8
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Yes thats correct. I was making refrence to his mechanic pointing at #5, insisting it was #4. The placement is wrong, but the mechanic'* tech tool {or whatever he used} may have seen it fire on 5, when it should have fired on 4. Know what I mean?... Another words, it could be a simple cross fire, wrong firing order or plugs. Thats why I suggested to closely check those things. Sorry for the confusion...
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Old 04-29-2004, 04:39 AM   #9
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Wow, talk about a response! First, thanks to everyone for your input - it is all appreciated.

Let me touch on a couple of things I mentioned initially:
1) DTC 301 and 304 do refer to misfires in cylinders #1 and #4 respectively. My first thought also was a bad coil. The coil for cylinders #1 and #4 was replaced with a new (known good) unit - the problem persisted. Subsequently, I swapped each and all of the coils with new units not once but twice; each time the car was test driven with the codes ultimately reappearing. The ignition control module was swapped not once but twice with two known good units, also with the original coils as well as the two new sets of coils. Same result of either DTC 301 or 304, also 401 would occasionally surface.
2) The EGR has been replaced twice with new units. DTC 401 still codes occasionally.
3) The plugs are new, properly gapped, and have been checked by switching to non-paired cylinders. If the problem were one or more bad plugs, the problem should have travelled to a different cylinder when the offending plug was moved. I even removed all of the plugs and randomly reassigned them to different cylinders. However, the problem did not move. The plugs were even replaced with another new set. Still coding DTC 301 or 304.
4) The same thing which was done with the plugs was done with the spark plug wires. The wires are newer (maybe 10k on them) and were returned to the retailer under warranty and replaced. Still coding DTC 301 or 304.
5) My OBD scan tool did not show any out of spec data during real time monitoring or during capture mode. However, I will mention my scan tool is not as advanced as GM'* (obviously).
6) The GM service tech was explaining to me that he made the symptoms go away by tapping the injector (mysterious "tap test") for cylinder #4, but he was pointing at cylinder #5 while talking. I corrected him that he was pointing at the wrong cylinder, but he insisted that he was pointing at #4. During his entire explanantion (not very enlightening) he referenced his scan tool data for me, clearly showing misfires in cylinder #4. For everyone out there who knows the position numbers of cylinders on the 3800, you will all agree with me that there is no way you can mistake the two. Sorry, but to make a mistake like that in front of a customer who knows a little bit is unforgivable. It didn't help that he was prescribing: full fuel system flush, replacement of ALL INJECTORS, and "CLEANING" OF INTAKE MANIFOLD GASKETS. He lost all credibility with me.
7) It is also important to mention that the car has NEVER presented with a "no-start" situation nor a sudden, unexplained dying of the engine. This leads me to believe the cam and crank sensors are still good. When the codes are not lit, the car runs like a champ: great throttle response, appropriate functioning of the TCC, and good economy.
Fuel pressure tests right where it should be according to the shop manual.
9) The engine has had a thorough visual inspection of the wiring harness and vacuum connections for any external signs of breakage or damage: nothing found.
10) The car has always been fueled with name brand gasoline from only a few local stations known to have new fuel storage tanks. The last time the fuel filter ws changed, the old one was cut open and no appreciable crud was found.

I believe I have eliminated the plugs, wires, EGR, PCV, air filter, fuel filter, fuel pressure, coils, ignition control module, crank/cam sensors, and O2 sensors as possible sources of the problem. Whatever the problem is, it is intermittent and not consistently reproducible (sounds more electrical rather than mechanical). Sometimes it codes immediately after clearing the codes, sometimes it will go weeks without a code appearing.

I am beginning to suspect the PCM as a possible culprit, since I have covered most of the common sources of problems according to the shop manual diagnostic flow charts. The other confusing thing is that even though the plugs, wires, and coils check out fine on cylinders #1 and #4, the plugs BOTH show excess carbon deposits (sooting) to an equal extent. The PCM is the next step up the ladder in the chain of events controlling combustion and firing of cylinders. If everything else checks-out fine on cylinders #1 and #4, it'* starting to look like the PCM may be malfunctioning.

The other thing that leads me to believe the PCM is at fault is that the PCM on my 1998 SLE (also with about 60K on the clock and the same mods/maintenance) was found to be faulty: it coded DTC 301 consistently and predictably under full acceleration when the RPM'* exceeded 4000 (e.g., merging on the freeway). All of the above-listed items were checked on my SLE as well, but no cause was found. The dealership tech (different guy - no longer at the dealership) went through the stepwise diagnostic listed above and came to the conclusion that the PCM was the next step (he openly admitted to being stumped). After a little soul-searching, the dealership admitted to the PCM being covered under warranty for 7yrs/70,000 miles. Has anyone noticed or heard whether 1998 was a "bad year" for PCM'* with GM/Pontiac?

However, the current tech is convinced the PCM is not at fault on the SSE and the dealership will therefore not swap the PCM under warranty. I have a spare PCM, but I am concerned over the mandatory programming of the PCM for performance. Is anyone aware of whether the swapping of a PCM on a Series II 3800 requires dealership computer programming facilities or can it be done successfully at home? If I swap the PCM'*, is the car still drivable? Can I swap the PCM'* back again or will I have lost the programming?

Thanks again in advance for your thoughts.
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Old 04-29-2004, 09:46 PM   #10
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If its a new pcm....u have to have it reflashed...a used with the same year ....maybe not...i am contemplating a new pcm for my 97 bonneville...am going to try a new prom chip first....97 was the last year for replacable chips....any one try a performance jet chip? my next step is a new pcm...might try a used one first....
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