Didn't realize the edit of my last post would not move it in teh Que.

 

The way I do a leak down test is with an old sparkplug that I have brazed on an air fitting. This also makes it nice for changing valve springs without pulling the head. Put air to the cylinder and listen for air escaping from various places. If air leaks out the throttle body, your intake valve on that cylinder is leaking. If it leaks out the crank case vent, your rings are bad. If it leaks out the exhaust (or rather you can hear it leaking out the tail pipe), your exhaust valve is bad. If you see bubbles in your radiator, the head gasket has failed.

 

Thanks for the tip. I was able to consistently get a sound of air coming out of the throttle body. I was careful to try and stay at TDC. I also took the valve cover off but did not see anything obvious.

As I think back to when I was changine the LIM gasket, there was a lot of carbon build-up. Maybe this was coming from the EGR, but could it also be from a situation where the intake valve is not closing all the way and I am getting blow-by the valve back up into the intake manifold.

I'm going to re-run the leak-down test to be sure, and if consistent, start to pull the head

 

Latest update:

Pulled the bank 1 cylinder head. No major problems noted, however lapped all of the valve seats. Cylinder 5 was the worse. Could see on the exhaust valve that 5 was not running as hot (darker color). After re-assembling and starting, the misfire was gone. Drove around for a while and did notice some misfire on various cylinders. Not sure if some is normal or not. It was misfiring so bad before that the Tech 2 graph just kept scrolling. Let it idle for a while and there was no misfire at all. Several minutes, then all of a sudden cylinders 3 and 5 start misfiring and the idle is extremely rough. Looked at the fuel trim and O2 sensor readings and Bank 1 STFT was at 50% and LT was at 25%. The O2 sensor was showing lean and was at 95 mv and not moving much. Commanded open loop and the truck idled much better. When it went back into closed loop it ran rougher. Was still misfiring in open loop.

Let vehicle sit overnight and went for a short ride. Again ran quite well when cold, and upon returning home, idled without misfirng.

Let sit for about 6 hours or so and went out again, no misfire when I started out, but after returning, cylinder 5 was missing quite bad. Check o2 sensor readings and Bank 1 was at 67mv and the STFT was at 50% and LTFT was at 25%. Cleared fuel trims and noticed an improvement in idle quality although there was still a miss.

Pretty confident that this is not mechanical at this point, but back to the original suspicion of fuel or maybe electrical (Temp related ??) Also now thinking maybe a bad O2 sensor or plugged one from all of the misfire activity that has been going on.

Any thoughts on next step to try and pinpoint the problem?

 

do you have the updated fuel injectors in it?

 

No. Original Poppet valves as far as I know (160k miles). Was planning on it, then saw low compression and took care of that with some valve lapping.

Originally was trying to sort out whether it was possibly injectors or fuel pump.

Wanted to avoid just swapping out the parts, or at least starting wth teh most likely as both are quite expensive. The O2 sensor reading got me thinking there might be something else going on.

 

i would do the spider
the low compression reading could be from fuel washing the oil off the cylinders

 

I would run a fuel pressure test and fuel pressure leak down test first before replacing the spider. If your fuel pressure is borderline then injectors that may just need a good cleaning may end up sticking occasionally with the low pressure.

If the fuel pressure leak down test shows a quick drop in pressure after the pump turns off then there is the possibility of a fuel leak from the FPR into the intake and it is most likely to flow right into cyl #5 where you are having the most problems. Though with fuel trims being so high I would lean more towards a leak as the culprit then sticking poppet injectors. Well, unless they are sticking open. Usually they are stuck closed though.

The extra carbon on the valve of cyl #5 also could be attributed to a fuel leak as that cyl would be running very rich.

Cleaning and testing or replacing your O2 sensors is also a good idea as a faulty one or both could be sending the wrong signals to the computer and screwing your fuel trims which at those levels can cause a misfire as well.
As you said clearing the fuel trims made the idle better this could be the culprit. Though as always there could be more then one problem at work here.

 

This is the data that I had from just prior to pulling the head. I have not reconfirmed the readings. Never really figured out if this was sufficient or not.

Fuel Pressure reading clarification
1997 Blazer with 160K miles. Misfires at idle terribly. Swapped out all electrical (Coil, Cap, Rotor, Plugs, Wires) except for the Ignition Module. Distributor gear looks good (Had it out for an intake gasket change). Not much play on the rotor. When I watch the TECH2 misfire screen it is typically either cylinder 3 or 5 (Usually sets a P0300, however). Seems worse when car is hot. Misfire will lessen / quit if I rev the engine at idle to about 1500 rpm.

Trying to decide if the fuel pressure readings are OK or not. Followed the book method post. When the pump primes and the pump is running, I am at 60 psi. After the prime, it drops to around 55, then seems to rise slightly to 56, sometimes 58. When idling, the pressure bounces a bit but is at 52 -53 psi. If I stab the throttle, it'll jump to 60 psi, then return back to the 52-53 value.(Are these acceptable?) I did install a new filter, but achieved the same readings.

I also did an injector balance test and pressure went from 57/58 to 48 psi after the pulse on all injectors, so I could not see any difference. Trying to either rule out the fuel pump or not. Thinking it may be the spider assembly based on many of the posts.

 

hi, can you tell me how much play your rotor has? like if you pull off the the distributor cap and then grab the rotor and turn it left to right, how play is there? sometimes you can put a ruler near the edge of the rotor fin and measure it going back and forth. mine measured 1/32" of an inch. how much is yours? thanks