First off, sorry for the length of this post:rolleyes:

I have been chasing this issue down for years and finally fixed it this weekend. I wanted to share what I did b/c this seems to be a very common problem in the 1999 Yukons and Tahoes. I have spent a ton of time on this AWESOME forum trying to get input and wanted to share my cure in case it helps someone else.

I have a 1999 GMC Yukon with a Vortec engine and 130,000 miles. My issue first showed up at 35,000 miles and has shown up from time to time since then, getting progressively worse.

Sometimes, the Yukon would start to misfire when towing and occasionally when just going up a hill around 70 mph. The truck would start to shudder and shake and even if I let up on the throttle and then tried to accelerate, it would happen again. If I didn't let up onthe throttle, the service engine soon (SES) light would flash and then go on solid. At this point the truck would smooth out (it seems to go from a default mode for fuel and spark).

Running the codes would get me a P0304 misfire on cylinder 4 and other times I would get a random misfire.

I have had it to several mechanics and run scanners on it trying to isolate what the issue was. It was difficult to get this problem to replicate itself which made diagnosis very difficult.

The following are some of the repairs I have done (many not related to this issue) that have not fixed this problem:

1 - Multiple full tune ups w/ AC Delco parts (cap, rotor, plugs, wires, fuel filter)

2 - Replaced fuel pump b/c fuel pressure was marginal

3 - Replaced intake manifold gasket (due to leaking coolant)

4 - Replaced fuel injection spider / poppets w/upgraded version

5 - Replaced distributor due to issue w/the rotor mount

Two weeks ago, I was driving down the highway and came to a hill that tripped my SES light for what seemed to be the 50th time.

This time, I decided to replace the crankshaft position sensor. It is one of hte easiest pieces to replace on your Yukon. it is located right behind the harmonic balancer at the front of the oil pan. All you need is remove the wires from the harness and an 8 mm socket and ratchet to remove the one bolt holding it in and you're done. 10 minutes max.

My Yukon FINALLY runs PERFECT!!!! :bananasmi :bananasmi :bananasmi

I even towed a car on a car hauler the past weekend for 40 miles down back roads, the highway, up hills and everything without even a minor hesitation or shudder.

You should obviously rule out the other issues such as a fuel pressure and spark issues (cap, rotor, wires). If none of these issues are the cause of your problem, it may be your crank sensor.

Hope this helps someone else.

That is some awesome info and I will definitely retain it for future reference!! Thanks for sharing!

By the way, P0304 would be the code for a number 4 misfire.

That is some awesome info and I will definitely retain it for future reference!! Thanks for sharing!

By the way, P0304 would be the code for a number 4 misfire.

Thanks, I updated my post.

Replaced distributor due to issue w/the rotor mount what was issue??? i am hearing a ticking noise coming from my dist after doing the intake manifold swap. and would you happen to know if there is a wire someplace to take loose to time it??? thanks

When you say your fuel pressure was marginal, how far off was it? I'm running about 50-55psi KOEO, and I've pretty much replaced everything you have, and more. I've got the Chevy techs baffled as to why it is happening, so I may give this a shot.

What's the easiest way to get to it, underneath or from above?

I too have this issue. The truck runs great without a load. Put my 3800lb boat on it and it starts bogging down and trips the Check Engine light. Codes are mostly "Random Misfire". I also had the MAF sensor code show up once. All codes go away after driving the truck without load.

I have replaced several things, but will give this a try. This is a 99 as well with only 110K.

Recently replaced:
cats(Went out)
full tuneup(Thought it would help)
fuel pump(Went out) and fuel filter.

I have not had the fuel pressure checked. Figured the shop I had the fuel pump installed at would have done that.

Hopefully, this will work! I need to get the boat out for some fall fishing.