tex50, you are not alone.

I just bought a good condition 98 Ram 1500 with 5.9 and 96k on the clock. Since I know the seller, can verify that engine was properly maintained and not dogged. Noticed a slight rough idle when cold. Then PCM threw a P0308 code during a 250 mile trip. Now engine idles noticeably rough with trans in drive and foot on brake (hot or cold). Engine smoothes out when I put foot on gas. I have checked the following:

#8 plug. Plug was stuck when I tried to remove it the first time. Took lots of patience and PB Blaster to get it out. Wound up breaking off the insulator in the process. Chased the hole and did a quickcompression check beforeinstalling newplug. Cylinder pumped up to over 120 psi (No WOT or all plugs pulled). Assumed it to be a good cylinder. Reconnected everything. Same Code.

Checked #8 injector. Injector is operating (audible clicking). Injector coil resistance within spec. With engine idling, pulled connectors one at a time for each injector on right side. Noticed immediate RPM change for #'s 2, 4, and 6. No noticeable change for #8 however.

Re-routed plug and coil wires per Dodge service bulletin regarding 5.2/5.9 crossfire issues. Cleared code, but came back after several miles.

Suspected a low voltage condition so I pulled the alternator and had it checked out. Turned out to be good with plenty of brush left. Battery is only 1-1/2 yo and cells checked out with hydrometer. Charging system produces 13.7 volts at the battery posts at idle.

GOING FORWARD: Will check cap and rotor, throttle body, and IAC. I will also perform full compression test with all plugs pulled and WOT. I'm now worried that the #8 cylinder has the dreaded crack between the valve seats. If this is the case, what would the symptoms be? Wouldn't the engine run rough through the entire RPM range?

Any help the forum can offer would be greatly appreciated. Tom

 

did you disconnect the neg side of batt? if not try that and see what codes you pull then.

 

dodgewilltravelYes. The CE just came back on so I will check it frist chance I get.

 

Great.. Never saw that tsb before and my wires are not routed close to that. Guess I'll try that next. Could this be a problem that starts out of the blue. Been 100k miles with my wires routed like factory. Thanks.

 

Here is the tsb link:

http://dodgeram.info/tsb/1998/18-48-98/18-48-98-v8.htm

Making this change had no effect for me.

 

Hmm! #8 CYL. Besides wires, it's the plenum or cat. #8 is closest to cat and will get the most heat if the cats bad. And most plenums that are gone seem to blow on that side towards the #8 cyl as well.

 

Ok, just went to check the PCM after the battery cable was off. Still only P0308.

 

zman17, thanks for the heads up. I will check this out. I've noticed many on the forum have HP cats and exhaust systems. May need to upgrade as well.

 

Catalytic converters don't cause misfires. They can clog and choke an engine, which sometimes will throw MAF codes because the PCM sees too little airflow when compared to engine speed and throttle position, but that's about it. And the plenum gasket would have to have one helluva leak around the #8 port to cause a misfire. That would be easy to find with a can of carb cleaner.

tdrex-

pull the valve cover and make sure the valvesprings on #8 are OK. I've seen the magnum motors break a spring close enough to the seat or retainer that they were still able to close during a compression test, but did not snap shut fast enough when the engine is actually running. Also, just because your injector is firing does not mean that it isn't faulty. Best way to test for that is to takethe injector out and swap it with another hole and see if the misfire moves.

 

I know the cat will not cause a missfire, but if its been clogged for a while the heat will destroy that plug . And if the plenum has been leaking a while it will also ruin the plug. However , he did say he replaced the plug, oh well.