Hi, All. Apologies if this is well-covered elsewhere; did a little searching and didn't immediately find something specific. Thanks in advance for any help, and sorry this turned into a book.

Questions:
1. Could my P0300 be caused by coolant filling the spark plug access holes and getting inside the COP boots?

2. How hard is it to change the intake gaskets? Do I have to pull the entire intake? Can that be done with the engine in the truck? Is there a good instructional thread somewhere?

Vehicle is a 1999 F350 4x4. 6.8L V-10 (believe in this year that would be a 2-valve). Mods from the previous owner include cold air intake and a set of exhaust headers. Unsure of any others off the top of my head.

Symptoms:
1. Coolant Leak: Several months (and about 2,000 miles, mostly towing) ago, noticed that each time I started and moved the truck around the yard, when I shut it down I smelled anti-freeze. Didn't seem to be using any significant amount. Eventually tracked it down (I think) to a part of the intake manifold that carries coolant, which appears to be leaking around the gasket at the front end of the right-hand head/valve cover area. The smell seemed to be coming from the exhaust Y-pipe, where the coolant was dripping off the back end of the head and down the starter and evaporating off the exhaust when it warmed up.

2. Misfire: A couple of weeks (maybe 50 miles) ago, the Service Engine Soon light came on. I had not yet noticed the engine running rough, but it immediately became evident that it was missing on at least one cylinder. My ScanGuage II showed P0300 (Random Misfire). Have played with driving it a few more times. Usually when I first start it up, it seems to run fine for a 10-20 minutes (or possibly until I first get it up to highway speed), and after that I have a pretty serious misfire. It's worst around 1500-2000 rpm. If I get on the throttle, it seems to clean up somewhat, but not perfect. (I liken it to an old multi-carburetor Honda motorcycle when it starts to run out of gas and you can get it to run by opening up the throttle further).

Troubleshooting:
Last night I started pulling COPs and spark plugs, hoping that I'd find a fouled plug to point me in the right direction. I pulled 8/10 (haven't tried for the rear two on the passenger side yet). Plugs all looked reasonable, and most looked great (nice and dry, like they'd been running fine). BUT the front three on the passenger's side (where the coolant is leaking) all came out with the boots damp and the plug threads wet with coolant, presumably because the sumps were partially full of antifreeze.

 

Instructional how to on a 5.4 intake manifold replacement.

2004 F250 Intake Manifold Replacement

Sounds like the coolant leak is related to the misfire. I just purchased new Denso boots kit and extra COP from Rock Auto for around $75 shipped. Something to look into.

Autonation White bear lake, Horizon parts guy and rock auto will probably be best on parts pricing. I would not wait to fix the issue as it will only get worse.

 

Thanks for the link! That is helpful, and the photos give me something to go back and compare with on my 6.8.

 

I would take a good look at the coolant hoses on the top of the manifold. They are on front passenger side. They could be the issue instead of a manifold gasket.

 

Thanks for the tip, rock2610d. I actually went ahead and pulled the intake today, since I was pretty certain I had one or more coolant leaks in those gaskets. Ordered the gasket set, and will try to put it together later this week.

I will take a look at those coolant hoses to see if they could be the source or a contributor.

I'm hoping that getting rid of the coolant in the spark plug wells will clear up my misfire. But while I've got nice access to many of the vacuum lines, does anybody know if there are particular ones that can cause misfires in the 1500-2000 rpm, moderate load scenario?

 

Well, I got the intake back on. Good news is she still runs. Bad news is the misfire came back in the first 20 minutes or so, while driving on the highway. Any ideas on what causes a misfire that only shows up after the engine gets good and warm?

 

Did you replace any cop's or boots? Usually, once a cop causes a misfire, it is dead and no way to bring it back.

 

I didn't replace any yet. I had looked at a few of the springs on the ones with the most coolant in the wells, and they all looked decent, so I just put it back together to see. I'm currently working on getting my Scangauge II to read cylinder misfire counts, since the truck isn't throwing any codes right now. Hoping that will save me some time, since the misfire doesn't usually happen at idle right after I start the truck, so it'd take a long time to round-robin a COP to find the bad one(s).

 

Time for another update. I heard back from Linear Logic (Scangauge mfg), with the numbers to plug in to read the misfire monitor data. I *think* this is the $53 data from mode $06, but not 100% certain. It does seem to jive with what I saw on O'Reilly's Bosch scanner this morning. Both sources show a non-zero misfire rate for cylinders 5 and 6, so that's probably the next place to attack. Got a friend with a known good spare COP, so I'll probably try replacing #6 (easy to reach) and see if that changes anything.

I put in a new thread with the info I got from Linear Logic, in case somebody else needs it. Link is: https://www.ford-trucks.com/forums/1...l#post16265325.

 

Solution: Finally got the truck running right this evening. After replacing all 10 spark plug boots (since Rock Auto had a set from Denso for 30 bucks) and moving the two coils on the misfiring cylinders, the ScanGauge showed the misfire followed the coils. I ordered a set of 10 from ARA Parts, on Amazon. Only replaced the three that were showing any misfires for now, and I can no longer get it to misfire. We'll see how long the off-brand coils hold up. Hoping for the best.

 

Denso coils on Rockauto have been reported as re-branded Motorcraft. They are only $23 too.

Glad you are getting it sorted out.

 

Glad you got things figured out.

I opted for really cheap coils, I mean really really cheap, 10 ebay coils for $45 or so. It's been 2 years and 9000 miles with no issues yet.

Plus they have a life time warranty for what ever that is worth.....I think I will just replace all 10 again when I have issues. I could do that almost 10 times for the price of 1 set of OEM.

 

I have been running the ARA coils for 3 years now. So far so good.