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1999 Yukon - Finally fixed misfire under load at 70 mph

09-09-2008, 12:57 PM
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.

09-09-2008, 01:34 PM
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.

09-09-2008, 03:20 PM
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.

12-06-2008, 09:04 PM
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

09-04-2009, 07:04 AM
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?

09-08-2009, 02:55 PM
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.

01-18-2010, 07:14 AM
Man I wished i would have read this a year ago! I've replacedeveything from the fuel pump to catylatic coverter. I recently replaced the battery, which had a bad cell and the yukon ran fine for a month. But now is running rough again. It runs rough under a load or even while idling/in gear. it is still a great vehicle. 163,000miles and still looks fairly new. I'm going to try this crankshaft position sensor, I hope this is the final fix. It's the wife's vehicle so you know it is top priority!!!!!

06-11-2010, 01:31 PM
I have a sim problem. Its runs rough at idle up to 1500 rpm and then smooths out. I am showing a code ofcyl 6 missfire. I have performed a tune up with plugs, wires, cap, and button. It went away and I cleared the code. Ran fine for about 2 months and now its back. I pulled the plug, and a couple of others to read them. It was a little browner then the others, but not fouled, and very clean. I performed a compression check and found that side to be 150 to 170 across the bank. It ran good for about 10 min and then went back to skipping. I am also showing the O2 sen might have a problem, as no code it set but it is showing yellow on imissions. I wonder if this sensor could be bad on this. This is a 5.7 (350ci) with 222000 miles on it.

Just pulled the CPS out and cleaned and checked voltage to the unit. The voltage was 11.5. by the book, that seems fine. However I dont have what is needed to check the sensor. But now I am getting cyl 2,6. misfire. I will change it on monday and see how it works.

06-13-2010, 04:17 PM
I did the whole tune up thing as well. Did not help. Replaced the fuel injectors and it runs smooth everytime.

06-16-2010, 12:37 PM
I changed the CPS to no avail. the only diff now is that it went from Cyl 6 to Cyl 2 missfire. I can reset the codes, but I get a flashing CAT,EVA,o2s,htr,and EGR, But I dont know how these would allow the skip at low RPM and then smooth out a 1500 and Higher RPM. Fuel injectors would cause a miss on many cyl I would think, but I just dont know. Please Help!

06-16-2010, 03:06 PM
I only had one cylinder showing a misfire as well. I would also get other codes that popped up including the MAF sensor. I am not an official mechanic and it sounds like the same issue could be caused by many different factors. Mine only happened under load. All the misfires made my Cats go out so I would refrain from driving much until it's figured out. I tried the crankshaft position sensor as well, but it did not help in my case. New Fuel Injectors have been on since Jan. and no problems whatsoever.

10-04-2011, 10:37 PM
I had a similar problem with my 1999 yukon, turn out to be the gear on the bottom of distributer. The gears were worn to a knife edge after 100,000 mile I was getting misfire codes on differnt cylinders. This seems be a common problem on 5.7 gmc 1999 engines.

04-29-2013, 05:20 PM
I have a 1999 gmc yukon got the truck cheap last week $1500.00 had a seriouse miss ,checked compression on #1 cyl was 30 psi,#3 was 180 but misfiring .just replaced the head found out it was cracked. now still have code po303 bank 3 misfire.all new gaskets.thinking maybe an injector but now that i read this possibly going for crank sensor.all new wires dist cap plugs,head .any comments

04-29-2013, 06:28 PM
I have a 1999 gmc yukon got the truck cheap last week $1500.00 had a seriouse miss ,checked compression on #1 cyl was 30 psi,#3 was 180 but misfiring .just replaced the head found out it was cracked. now still have code po303 bank 3 misfire.all new gaskets.thinking maybe an injector but now that i read this possibly going for crank sensor.all new wires dist cap plugs,head .any comments
I replaced two cracked heads on my 99 yukon. After running engine condensation in engine cause my distributer to rust up and filled my cap and rotor full of rust dust causing miss fires.There is a small screen in bottom of distributer make sure it is clean. And yes I did change all fluids before running.While you have the cap off check the distributer by rotating rotor with hand it should not move if it does gears are worn on bottom of distributer causing engine to set off miss fire codes. It may be as simple as a bad plug or spark plug wire.
Good luck

04-29-2013, 06:50 PM
I also just bought a 99 GMC Denali last week and the guy who sold it said it needed new spark plugs and maybe brakes but my mechanic got a misfire code and he's telling me it needs a new fuel injector spider and that might not even solve the problem. It's a hit or miss. The part and the job is like $500 and it might not even fix it?? This man has over 25 years of experience under his belt. Idk though. The truck runs ok, it just sounds like shit! Any suggestions?? It's not even eating up my gas all like that with the existing problem. :uhoh:

04-30-2013, 05:26 PM
thanks for that help gimpy will do. did a noid test today on the injection system elect.all is good waiting for ebay to send me 1 new injector for $29.99 and free ship.cant afford a $whole spider setup for $300.00 ,pulling that spider system is quite simple(for bullseye inquiry) worth saving $500.00 on a mechanic that doesnt know shizzl.a couple of bolts .hoping injector is what it is for bank 3.also replacing coil for my spark is yellow ,should be blue.

11-12-2014, 08:08 PM
I have also been subjected to this tortureous problem. I tried applying all I know of this and logic really did not seem to apply. Every troubleshooting technique used continually came up wrong!!!

I went through all of the replacement stuff that you did also. One after another, the problem still would popup whenever I would hit around 65-70mph under load. After a while it seemed that it was actually creating other issues with the Vehicle. And sure enough, after one episode where I was on a highway in Upstate NY and was going over a mountain, My Tranny went!!!! ( I really think the bucking finally took it toll on it). So I was in a jam. Replace Tranny? Was this maybe the cause of my problems? The Truck is in Awesome shape ( Afterall, I replaced everything!!!! ) So I did the Tranny!
Nice New Rebuilt Tranny from PDQ in Texas. Got it all put back together, and test drove it around town a bit. Felt a lot different as far as shifting and all that. First Strain test , 70 mph on a hill I got SES light coming on!!!! GRRRR.

Codes say MisFire and this time 2 O2 sensor errors. WTH? Then I recalled the previous year having a rash of O2 sensor codes that disappered. So I decided to replace all four! SURPRISE!!!!!! After that , I took it out full load and no Error Codes, smooth acceleration, and drove awesome!!!! HOWEVER, That is when I noticed a New Issuse! The Infamous Vibration and or Shudder at 70mph!!!! WTH????

Had underside checked out, found cracked U-Joint mount on the Shank, rear driveshaft, coool!!! NOT!!! 200 bucks later, driving out of that shop, still had the shudder.
So I seem to have traded one annoying dilema for another!!! And it is funny that these are so Common on these Beasts! WHY?????????????

09-09-2015, 11:59 AM
Just bought 99 yukon with the anticipation of it being pretty much worry free reliable, my SES came on after slowly driving on a dirt road and maybe some really cheap gas I'll pull the codes today.. I really hope It's just injectors, I'll check the intake gasket as well.. meaning I'll replace it, but damn what a heart break after I sold my problematic jeep for something reliable.. I hope it actually is.

11-04-2015, 05:35 PM
I have had the bucking issue at speed. After hundreds of $ and changining the spyder to real injectors I finally found it to be the anti-knock sensor $60 and rather hard to get to. The ECU keeps advancing the timing until a knock,detonation, is heard then backs the timing down, does not due it on acceleration or deceleration, just normal cruising. Doesn't show up on normal scans from expert shops. I had to find this one myself.

05-02-2016, 10:40 AM
Thanks for the info. I was just about to change out my catalytic converters. I was having the exact same problems and after I read your posting I went ahead and changed out the crank position sensor and BAM problem went away. This saved me LOTS of money!! Thanks again.

FYI, I did not get any codes.

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