I have had and idle issue since i received this 85 c4 about 6 weeks ago. after extensive testing and research and trouble shooting I have narrowed the problem to 2 specific areas. 1- valve seal problem....the car sat for 11 years and i believe the valve seals have hardened. this leads to a puff of smoke at startup and possibly the idle problem. possibly a sticky valve.... I am thinking of doing a compression test.....if that is okay....2-maybe replace valve seals......if compression is ok......do you all think it is worth the effort to do the valve seals?........i just don't think it will be a bad cylinder....the car runs fairly well with and idle that runs 640-590 rpms at idle.......the car accelerates fine.....runs smoothly at highway speeds....only 45k miles........is this really a problem. the car was given to me and maybe I have $1500 tied up in it. it looks like new..the passenger seat does have some wear in the leather...should i just let it be...maybe run some mystery oil through it .....and see how it goes? even the air is so cold it causes problems with fogging on the outside of the windshield. i love this car. but am i being overly obsessive on a 27 year old car that would appear to run great for the average person? what would you guys/gals do if it were your car?

cmj

p.s. corvettes are just as bad as drugs!

 

The first question is are you mechanical? Can you turn a wrench? If you can, then replacing the seals is not too difficult. You can replace them without taking the heads off. You can use the "rope trick" where you pull the spark plug and then stuff the cylinder with a thin rope and then turn the crank very slowly so the rope presses up against the valves. You then compress the spring and remove the retainer. Pull off the spring and then grab the seal and pull it up off the valve stem. Then push the new seal down over the stem and seat it on the head. You can use compressed air instead of the rope. Suggest you use Fel-Pro intake seal for both the intake and the exhaust.

Check for any vacuum leaks that may alter the idle. Next check the TPS to be sure that it is set to .54 volts. Next would be to remove the IAC valve on the throttle body. Then remove the TB and turn it over. The IAC screws into a section under the TB. The IAC opens and closes and lets air into the TB at idle. The passages in this section often become blocked with carbon build up. You can remove this and clean the passages.

 

if you do the valve seals, which is not that difficult (see above post) ,as you rotate the engine (by hand with the plugs out) verify all the valves go down the same amount) a valve which goes down less than the others is a sure sign of a bad lifter or bad cam lobe (probably both), a condition which may or may not show up (or may be mistaken for other maladies) when doing a compression test; then do a compression test. If all is okay, new plugs, new spark plug wires, new rotor and rotor cap plus a new fuel filter, air filter, oil filter and oil, should make for a really nice running engine. good luck and enjoy.

 

I have done many valve seals. I recently had a scan done on this vehicle and the mechanic reached the same conclusion as I. No codes or leaks, but and idle that was not quite smooth. The mechanic stated that a compression test would give a better indication of a problem. He also stated that due to the car sitting 11 years, that oil may have gummed up the valves and caused a deposit on a valve/valves and/or the valve seals may have hardened, which causes the puff of smoke after overnight starting. He also suggested a cleaning of the valve train by a chemical process he uses that might clear the problem. I am unfamiliar with with this cleaning process which involves places solvents in the valve train and sitting overnight than (not sure) and then removing solvent and changing oil and filter. ($100) Have you heard of this? The 11 year layoff did not help this car...today I found the heater cable stuck and I am in the process of tearing the dash apart to repair the cable. In the process I removed 2 basketball sized clumps of mouse nesting! Yikes! Can't believe it runs fairly well! Anyway, thanks everyone for the info!

 

Valve seals is very common in these L98 engines. So much so that many folks leave it and live with the small puff at start up and enjoy the lube thats added at cold start. Its not a big deal and the valve seal would have to be GONE to effect the idle that much.

You need injectors. Plain and simple. These injectors were trash to start with and after sitting I can assure you that they ARE gummed and not flowing well.

Forget that cleaning process. The shop is trying to "sell" you on something. Your suspecions about solvent remaining are valid concerns. It dont take much solvent to ruin the oils ability to lubricate.

I'd run some good quality injector cleaner (gonna cost $20 a bottle) thru the thing, and see if that does not help some....I'm sure that it will. This will also prove the injectors are at fault. Injectors don't generally wear out. They get dirty and require cleaning off the car. These inj DO have problems with the coils shorting and getting too weak to keep up with the fast firing at high rpm.
Its only $250 to replace the set and a Saturday afternoon to do the work. Call Jon for the details. he'll walk you thru the job.

Look at all vac lines, make all the necessary adjustments, and by all means, IF you plan on doing any work yourself, order a FSM NOW so you have some idea of what to do and why. Don;t waste your money on a Childrens or Haynes manual for $19.95. Those are doorstops. A real FSM is going to be close to $100 but its priceless with the facts that are stored in the 2 book set.
You;ve got a jewel there...that age with the low miles...quite a find. Treat it right and it will deliver many yrs of fun and satisfaction. Maint is the key to keeping your cost down on a Corvette. Do the maint and do it well. You get paid back by a car that does everything right.
Good luck !

I run Mystery oil thru mine every few months to clean the top end and add a bit of lube. It seems to help and run a bit smoother when this is done.

 

Before you replace the valve stem seals, I would do a leakdown test first. If the valves are not sealing, or the valveguides worn, a valvejob is needed. There is no sense in wasting time doing the seals.