Dirty Pistons

[PorterzSupra]

Just got a "low milage" 7M motor, anyway I took off the oil pan and took out the spark plugs and noticed the pistons were dirty, they have black carbon or somethin on them and I was thinking if I could maybe spay brake parts cleaner in the top so that it would liqufy the black residue and then I would turn the crank untill it leaked past the rings down into the crank area, and I have the pan off so it would just drip down and the rest would evaporate, of course I would re oil the piston walls via the top right after the cleaner had dryed. Do u think this would be okay to do? I know its not somethin that is usually done but I dont see how it could hurt anything. Does the carbon on the piston tops effect performance in any way? Its strange because #6 piston has a good deal and number 5 has little carbon at all and all the rest have moderate amounts. I dont know why #5 is cleaner then the rest oh well.. so would this be ok to do and does the carbon on the pistons tops effect performance?? I know I ask odd questions...

 

[Allan_MA70]

i would say it has/had a BHG as coolent steam cleans pistons nicely!

 

[PorterzSupra]

oh no, I dont wanna hear about the head gasket, come to think of it I think I saw a small a/f wetspot on the intake side of that piston when I took the manifold off =(

 

[Allan_MA70]

secondhand motors are secondhand problems, tear down and inspection is the way to go, if its all good put it back together with a hone, rings, bearings, arp studs and a stock gasket you will have a good engine to last you a LONG time

 

[PorterzSupra]

I ended up using the brake cleaner but had to suck the remainder out of the cyl with a vacume and small hose attatchment, it worked really good, guess gotta do HG though oh well... they said they did a comp and leakdown test on it and the HG was good... guess they lied lol. The cleaner dident drip down like I expected it to but I sucked it out nicely. BTW does the carbon on the tops effect performance??

 

[jetjock]

I'd rebuild it but if not you can either A) Soak them in a solvent like Berryman's overnight or longer B) Clean them with water or ATF (not recommended) injection after you get the thing running. Depending on where they are and how bad carbon deposits can affect compression ratio, cause pre-ignition, and effect closed loop EFI operation, thus causing emissions failures.

 

[IJ.]

Maybe I'm dim or just not getting it but you bought a second hand JDM motor to replace your broken motor and now you have a broken JDM motor as well?

 

[jetjock]

I'm the dim one and getting dimmer by the minute. I'd have to be just to answer some of this stuff

 

[PorterzSupra]

I replaced my 7M motor with rod knock for a low miles one nothings broken I just wanted to clean up the tops of the pistons. If I had all the money in the world id rebuild it.. wait no, if I had alot of money id say f*ck the thing and buy a viper =)

 

[IJ.]

If nothing is broken why are you messing around with it at all?

 

[Boostedstr8six]

Without dissassembly, I would try water misting to clean up the carbon build-up if you are really that worried about it. I don't know what the norm is but the engines I've cracked open all had carbon build-up, even my own after only ~800 miles of operation after a rebuild. IMO, if it's thick and wet looking then there is excessive oil entering the CC and that may indicate that a full rebuild is needed. Gasoline alone will leave a good amount of carbon behind as it is a somewhat dirty burning hydrocarbon. I'm sure you could run a gasoline engine fairly clean but it would have to be in a laboratory setting which is an alien concept compared with a street driven 7M, or just about any other street car. If you burned nothing but propane or LNG you could keep your piston tops pretty durned clean though.

IMO, a used "JDM" motor is at the most a band-aid if used "as-is". A minimal rebuild isn't very cost intensive and will save you from more issues in the near future, unless you don't plan on driving the car for more than a few more months. I took that path with my Supra and have learned the hard way.

 

[PorterzSupra]

yea, I agree but sometimes u get lucky

 

[atlpd3147]

Used motor= issues IMO
Take the head off buy arp head studs and a metal head gasket.
And if you really want to be sure you're ok rebuild the bottom with new rings and bearings. All done for about 1k or less.

 

[Boostedstr8six]

I got lucky with my used JDM. I drove the shit out of it (even hit 20psi with stock fuel/afm/ct26 one night) and never had any issues on the road. When I tore it apart it was on the verge of a major BHG and showed obvious signs of running lean on 4 out of six cylinders. The only thing that lets those engines take a little abuse is the lower miles (with an honest retailer). It doesn't last though and ends up costing.

Porterz,

I would rebuild the thing fully if you don't need the car daily and plan on modding. Make it happen or get rid of the car. It's taken me over four years to realize that and has cost me more than it would have to have owned a decent used, bone stock, BMW M3 (E36) daily driver/SCCA SoloII whore . And I still have a relatively mild MK3 with faded paint. (note the mildy insane dedication to the MKIII)