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Discussion Starter #1 (Edited)
Hello everyone, I'm dropping this post here because I need some guidance, and assistance with a random, and traveling misfire, hard starts. P0300 My truck is a 5.9L Gasoline


Currently my truck suffers a random misfire, a large amount of power loss, heavy shuddering at 1500-1600 RPMS under driving conditions, occasional stalling at idle, rough idle, intense smell of fuel, flashing check engine light. So here's the whole list of my own battle with this thing, of going from throwing parts at it, to trouble shooting. Previously I had thought this was from a vacuum leak due to the trucks loud whistling sound that appears when the truck starts to idle very roughly even stalling out at times. This problem would typically correct itself after the truck warmed up. So I went through and refreshed the vacuum system on the engine and 4x4 transfer case, giving it all new lines, swapping out the EVAP Purge Valve on the side of the engine bay, swapping the PCV. Sadly this did not fix it. So at my wits end with trying to chase this vacuum leak that I couldn't find because years prior I swapped out the intake plenum plate and gasket. I decided to drop it at the local shop. They later tell me that they couldn't find one, but are trouble shooting the misfire in the engine.

Thus from here the truck received

Spark plugs
ignition wires,
ignition coil
Timing chain
re-timed.
Compression test came back beautiful despite having 176k miles on it.
Idle got reset
New IAC
New Set of Injectors
The shop went through the wiring and said everything seemed ok.

They dropped me the truck and I went on my way. The Problem disappeared for a little bit, then came back even worse while I was 100 miles away. So I limped the truck back to the shop, and found out, the truck had some how fallen out of time, and its fuel pressure was all over the place.

So they dropped in a new Distributor and Distributor gear, they informed me that the cam gears looked good as well. Truck was retimed and received 2 new fuel pumps. Grabbed the truck and many months later went on my way. Problem still exists. I decide to hop back into it and trouble shoot the MAP sensors and Crankshaft position sensor. Checked for voltage at the truck side of both plugs and got good signal voltage. Then Back probed the MAP sensor on the front of the throttle body, and got readings that it was bad so I swap it out, ECU reset, problem is still there. Almost immediately reappears, so I figured I probably back probed wrong or got a bad reading. I'm at my wits end with this thing honestly, I debated seeing if the EVAP cans are clogged but don't have the money to just keep throwing parts at this thing. This truck also doesn't have an EGR Valve.

Also got good fuel pressure readings at the rail as well, unfortunately the gauge broke before I could get readings while it was idling rough.

Here are some readings I got from an OBD sensor during the trucks rough idling time.

While rough idling
Fuel System Closed Loop
O2 Front .0140 (Rough Idle) .0340-.680 (Normal Idle)
STFT B1 S1 -35% (rough) -5 (Normal)
02 Rear 0.020 (Rough Idle) 0.029 (normal)
STFT B1 S2 99.2%

The 02 Sensors do react when I give it throttle, but when I left off the forward one goes to practically 0 volts, If I'm reading everything right. The truck is going extremely lean when idling rough, and I can only assume its the reason its mis-firing and has no power. I've debated if its an out of wack ECU, a bad Crank shaft position sensor, or hell even an Engine rebuild. Unfortunately my diagnostic abilities are limited without a personal OBD reader, so I can't get readings while the truck is driving. I also might go in and recheck the forward 02 sensor to see if it fried.
 

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Discussion Starter #3
I did the entire Hughes kit, the plugs I don't know the shop pulled the ones I put in, then swapped them out. As for the distributor fuel sync, They did set the fuel sync or they called it the Cam/crank sync. Plugs in it are Champion RC12LC4 plugs opposed to the NGKs I had in it originally. I double checked the gaps on the ones I pulled and they where good, but I haven't gone around to all of them yet.

I disconnected my O2 sensors for giggles, and the truck runs much happier, and starts better albeit not perfectly. Check Engine light still flashes under hard acceleration
 

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If here is an issue with the cat/02's you would get a code/s. Might be somthing to look into. Cat's have been known to clog up but that usually effects acceleration due to the restriction. I never had any luck with Champions, Ran them when i first got my truck and would only get a year or two without a miss starting. Changed to Autolites and been running them ever since.
 

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Discussion Starter #5
If here is an issue with the cat/02's you would get a code/s. Might be somthing to look into. Cat's have been known to clog up but that usually effects acceleration due to the restriction. I never had any luck with Champions, Ran them when i first got my truck and would only get a year or two without a miss starting. Changed to Autolites and been running them ever since.
I had swapped my Cat about 8 months prior to it, I figured that it burnt up my 02 sensor because it was literally glowing. I swapped out the 02 sensor, warrantied the MAP sensor, then pulled the throttle body off and cleaned it completely clean, cleaned the IAC. Resealed it all when I put it back on, and everything suddenly started working. It still Idles like ass, but its not misfiring, and flashing under acceleration again.

Now my shuddering I believe its a different problem. The entire truck shakes violently. When the truck shifts into O/D at 45 miles per hour it drops to 1100 RPM, then at 60 MPH it does the same. Once its over 65 it drives perfectly, and happily. If I turn off O/D in the lower end it drives fine again. Ugh, back to the trouble shooting book for something different.
 

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Maybe get a spark gap tester and see how strong your spark is on each cylinder? I've done that to diagnose a weak spark on one of my cylinders on another car I had.
 
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