help, strange problem no compression cyl 7

[drdan131]

HI
My 92sc400 was running great, but, i started to hear the starter motor dragging. The car was running great up to this point. I had the starter motor replaced and a week after getting the car out of the shop. It developed a low rpm rumble and felt like it was running on 6 cylinders. Took it to the lexus shop this time, and they said to put back denso plugs which they did, ouch 300 bucks. ran great then 30 minutes later ran crappy low rpm rumble and running on 6 cylinders,. They did a compression check and found cylinder 7 no compression. leak down test air came out intake side of manifold. took to another shop and they said cylinder 7 also no compression. what is strange is that how does a car run great for a week, then run on 7 cylinders , replace plugs run great again and then no compression, something does not add up. lexus mechanic thinks i might have a bad intake valve in cylinder 7. why would it run good for a week, then lousy, then good. if its a bad valve it should never have run good.
one person told me to suck in some water into the intake because my valve could have carbon on it. tried it no luck. any ideas, spent 1300 bucks on starter and now my engine is blown, something doesnt add up.
you guys are experts, i've read some threads, hope someone can give me some ideas to try. dont think im tech good enough to pull the heads.: ive owned the car since 60,000 miles and its was running great until the starter replacement

 

[cribbj]

I would suspect that something got dropped into the intake manifold during the starter changeout. However, that it ran fine again with a new set of plugs would tend to disprove the dropped object diagnosis.

But a bad compression test, followed by a bad leakdown is pretty conclusive. Sounds like you at least need your heads freshened up, or just swap in a whole new long block.

 

[spf_lexus]

Adding to what Cribbj said, if your engine isn't burning oil, you can rule out piston rings and head gasket as a cause. Sounds like you might have a leaky intake valve. Possibly an ugly carbon deposit causing the valve to not fully close?

I serviced my 200k 1uz a few years back and I had 40% flow loss on a couple cylinders just from worn valve seats. I knew my mechanic wasn't lying because after the valve job, the hp/tq wa back and I gained a few MPG.

 

[drdan131]

Thanks I appreciate the input

Would it be better to pull the heads and
put have a valve job performed
or take a chance on a used engine
cost reliabilty
I've heard of inexpensive engines from
japan
or try to pull the heads myself
and pay someone to fix the valve
the engine has 240 k miles but I change
the oil every 3k miles and was expecting
to get 400 k miles out of it
anyone know of a good inexpensive mechanic
in the San Francisco bay area?
I love this car
thanks everyone
this is a great site and forum
thanks to lextreme for creating it
I've never found a car as smooth and
nice as my sc400

 

[drdan131]

Ps any way to get rid of ugly carbon

Hi
thanks again
any way to get rid of poss ugly carbon
deposit
I tried having water sucked into
intake manifold and also tried
seafoam
if I had to guess I have a stuck open
intake valve if that is poss
someone said to put quart of ATF
into engine to act as a detergent
then drain after 100 miles
wasn't willing to try this tatic
info wasn't from this site
happy new year everyone
wish I had the mechanical
knowledge to pull my heads

 

[cribbj]

For the cost of paying someone to pull the heads and getting a good valve job done, you're about 3/4 into the cost of a lower mileage junkyard engine. The only downside of a junkyard pull is that you don't know its history, and it may bring new problems to the table.

Still, given the high mileage that's on your existing motor, if it were me, I'd probably opt for the engine swap.

The problems begin when you open up these motors and try to do work that is as good as Toyota's original work.

 

[spf_lexus]

Yup, and most early UZ engines can be had for $500-$800. If you decide to get the heads done, thats going to be about $750 if you install/remove the heads yourself.

If a shop does everything, your looking at $2000-$3000. (the labor really adds up).

 

[gloverman]

Id be checking what has carefully happened. Kind of strange to suddenly have a leaking intake valve. I would be removing the LH tappet cover and checking for excessive tappet clearance and checking that the cams are in good condition.

Cheers

 

[spf_lexus]

Back to the cost, sourcing a set of oem cams plus suitable shim shicknessses are going to add up fast. Not saying a bad cam lobe didn't cause it but not many junk yards would be willing to sell half an engine. Pulling the valve covers is an fast/easy way to double check. I would think a weak valve spring could be a possibility as well. If a spring isnt compressing like it should, the valve wont close for the compression cycle, showing zero compression on a single cylinder.

Very odd issue nevertheless.