91 SHO mtx Severe misfire under accelleration

[91slofoSHO]

Hello I'm new to the forum. I just picked up a ultra clean 91 SHO with mtx for very cheap, with 117k on the clock, so I'm kinda expecting a challenge. After purchase, I notice there is no coollant whatsoever in the car. so after changing the oil(original looked very clean but used),filter, thermostat and adding the coollant I started it up. Seems to idle ok, a little steam in the exhaust but nothing like a blown head gasket. So I took it for a drive, and boy does it miss. It stumbles under hard accelleration at any rpm, no rythm to it and it feels very down on power so I'm thinking ignition miss. Then I go to change plugs, and all 3 front plugs have oil in them, the middle one actually had 2" of oil sitting in the plug well. There is also a more than noticeable ticking. The vehicle appears to have been well maintained and clean. I have a lot of experience with DSMs so of course I'm a professional mechanic (jk) but I'm looking for a good place to start diagnosing this motor. also the temp gauge doesn't work. Thanks for any replies!

 

[projectSHO89]

Oil in the plugwells is a really common occurrence and will cause misfires such as those you've reported.

New oil cover gasket set, plug wire set and plugs will correct that part of any problem. Might as well do the rest of the top end 60K while you're in there.

Steve

 

[SHO SPD]

Yeah sounds like you're in need of the 60k tuneup. You can search for all the needed parts, but the top 60k includes Valve cover gaskets which will come with the spark plug well seals which are leaking on yours. Spark plugs, valve adjustment, spark plug wires and intake manifold gaskets. If you do the work yourself it will be beneficial for you cost wise.

 

[Troywakeling]

Oil in the plugwells is a really common occurrence and will cause misfires such as those you've reported.

New oil cover gasket set, plug wire set and plugs will correct that part of any problem. Might as well do the rest of the top end 60K while you're in there.

Steve

X2

Mine did exactly what your talking about when I had bad plug well seals and oil 3/4s of the way up the plug in all six wells. I just did mine myself two months ago. It looks a litte overwhelming at first but its very very easy to do. Heres a great link with instructions http://www.shophoenixproject.com/upper60k/upper60k.htm took me about 5 hours to do but I took my time and made sure everything was right. This is what I replaced.

New valve cover gasket set, came with the six plug well seals $78
New spark plugs I used cheap copper NGKs... Some people wont use anything other than the autolite Double platinums but I didnt want the extra cost at the time and the NGKs work perfect. $12
New plug wires $45
3-4 cans carb cleaner $10
My upper intake gasket was fine so I reused it. $0
New thermostat $10
New coolant temp sensor $20
New 02 sensors $100
My shims looked a ok so I left well enough alone.

I did mine split up. Did the valve covers plugs and stuff first then a week later did all the sensors and thermostat. I plan on taking my intake back off before fall to tear the whole thing apart and really clean the snot out of it and replace all the intake gaskets and put DP autolites in. Make sure when you have the intake off to clean the egr nostrils to, If you dont have a code 332 yet you probly will soon.

If you dont have any oil in your plug wells I would start looking at your coil.

 

[projectSHO89]

His 91, unless a California car, won't have EGR.

Steve

 

[Troywakeling]

His 91, unless a California car, won't have EGR.

Steve

stupid atxs haha

Still a good idea to clean out that intake though.

 

[91slofoSHO]

Awesome guys thanks for the information! Yeah the oil is quite deep in all 6 plug wells. The link was very helpful as I didn't know what all is required with the 60k top end service. I will certainly be doing it myself as I don't trust any mechanics that I could afford!! I'll get on finding those parts and see when I'll be doing the service:thankyou: