I have been trying to diagnose a misfire issue. I doubled checked eveything today including dist. timing, rotor and wires, fuel pressure,vacum leaks etc. What really bugs me is that the truck starts as soon as you turn the key. After about 10 seconds running, the CEL comes on and flashes as long as the truck is running but, there are no codes. I unplugged a bunch of sensors and codes do register every time so, I ruled out the DLC. What could cause the light to flash on and off and not register a misfire code? On a side note, I looked at the timing chain cover from my previous engine and noticed that the crank sensor is still in it. I guess there was one already in the new engine. Could this cause a problem since there was no case relearn?
Don

 

Since you already have a thread discussing your problems, lets keep it there. The misfire is very likely being influenced by the other problems you are experiencing with your truck and keeping this all in one thread keeps from duplicating efforts/responses/etc.

*Combining back into original thread on the high idle issue and renamed the thread to include the misfire*

If a CASE relearn were needed, the PCM should trip a P1345 code for CKP/CMP Correlation Error. The FSM states that a CASE relearn should be done whenever the CMP, CKP, or any one of the links in the drive between the two (timing set, cam, distributor gear, or the entire distributor) get replaced.

 

OK will do and thanks.
Anyone have an idea as to why it doesn't throw a code?
Don

 

November 25 2010
Gmc Jimmy 4 door 4wd A/T.

This is to document all the steps taken to find the cause to a flashing MIL with misfire and no codes.

Fuel pressure was verified a few times over a period of a few hours. Results are:
key on engine off: 58 psi max. Tried cycling on and of but did not go any higher.
key on eng. run: 56 to 58 psi. Jerking the throttle open, pressure builds to approx. 64 psi.
key off engine off: after 5 min bleed time 55psi
after 10 min bleed time 52 after 30 min bleed time 48psi after 60 min bleed time 42psi
I realize that pressure is a few pounds below what it should be but, upon quick acceleration, it builds up to spec or very near so I don't think this is the cause of the misfire, or why the truck shakes and wants to die when put in gear,especially at idle which holds @ 1200rpm BTW.
Here is the link to a video I made to check fuel pressure:http://s1142.photobucket.com/albums/...t=100_0099.mp4

Checked the health of engine with vacuum gauge. 19"Hg @ 1200rpm idle. Needle very steady. Snapping the throttle brings it close to 0 and around 23 to 26 "Hg upon deceleration. Does not look like anything leaking or sticking or burnt valves or even ignition miss.Another link to video:http://s1142.photobucket.com/albums/n615/pappyam/?action=view&current=100_0099.mp4#!oZZ1QQcurrentZZ http%3A%2F%2Fs1142.photobucket.com%2Falbums%2Fn615 %2Fpappyam%2F%3Faction%3Dview%26current%3D100_0083 .mp4

Today, I performed a compression test that revealed cyl. #4 and 6 to be low (155psi) compared to others that are between 176 to 180psi. A second test with oil in the cyl. showed and improvement to 180 psi. Cyl. leak test confirmed that piston rings were at fault because air was coming out of the dipstick and leakage was around 25% on gauge. No air coming out of radiator or out of adjacent cyl so, no blown head gasket or cracked block. No air out of TB or tail pipe.
As stated earlier, this was suppose to be a new engine. Cosmetically, it did not show any signs of wear when I bought it although it had been sitting bolted to an engine stand in an unheated shed for the last 13 years. The PO took care to bathe it in oil. I was wondering if the rings could be stuck after sitting for so long!! I sure don't want to pull it out again. I poured about 4 ounces of a mix I made (30% PB blaster with 70% transmission oil) into each of the 2 low cyl. and will let it sit for a day or so to see if it helps. Anyone have another recipe to free stuck rings, if that is what it is? If someone has another idea of what the problem could be, I am interested in hearing it but, I am a little scared of how it looks right now.
Don

 

Well, I am really at a lost here. Zipped everything back together and started the truck. Real nice smoke show. The mixture I put in seems to have helped loosen 4 and 6 but, I still have that f*****g misfire. I narrowed it down to #3. Pull the wire and nothing chages. Idle's at 1000rpm now. Changed plugs this morning. Switched wiring from #5 to #3. Same thing. I have 178 psi in that cylinder and I know for a fact that the plug is firing. I have been at this for 1 month now and I need to get this on the road for next week. I don't know where to go from here. Please HELP!!!!
Don

 

I was checking the pulse from the vcm with a noïd light in the injector module connector when I noticed the pin for #3 cyl. was bent. Straightened and wisfire disapeared. Now truck runs like a champ except for the idle issue. I went for 50 mile ride yesterday to get it nice and hot. When I put it in gear, idle goes from 1100 rpm and jumps to between 1500 to 1800 rpm. The truck is acting like a thouroughbred and it does make it hard to keep on the line. I am sure it is something stupid and I am missing it. Like I mentioned in my first post, it ran good with the old motor. BTW, I put in new O2's so that eliminates a potential problem. As far as computer controlled equipment, the only non original part is the cranck sensor. It was already attached to the timing cover of this engine.
Don

 

You must have a vacuum leak somewhere for the idle to be that high... Glad you found the problem with the misfire. Those pins can be a pain to diagnose, but I've been there and done that.

 

I have looked everywhere I could. Went around the intake with a propane torch and no change in idle. There aren't that many vacuum hoses going to the intake. The ones that come to mind are brake booster, pcv,MAP,and the hose that comes out of the right valve cover and goes to the TB thingy where the air duct is attached. Is there a better way to check for leaks or do I have to unplug something while I am searching for a leak?
Don

 

Have you tried plugging all vacuum sources at the intake (including the EVAP purge solenoid on the passenger side) as well as fashioning a block off plate out of some sheetmetal for the EGR valve to rule it out as a source?

 

I did plug all vacuum sources except the egr. I will try this. I went for a ride with the scanner plugged in. I got code P0507 which is related to the IAC I believe. In the meantime, I remembered that when I cleaned the IAC,I plugged it in whitout installing it. By the time I got to turn the key on and get back under the hood to see if anything would happen, the pintle and spring had already flown inside the engine compartment. I found the parts and reassembled the valve but, it was difficult to put it back in. Is it possible that upon reassembly, the toothed shaft would not be sitting at the original place? The computer would activate the pintle but, it would not be where it should be or, would the computer adjust it where it should go anyway?
Don