So was going to do some general upkeep on car, but it was difficult to start and after extended cranking it did start. It ran a bit rough for a few minutes and the check engine soon light came on. Took it for a short run to see if I noticed any drivability issue and seemed to run ok. When got back home noticed some oil smoke, opened the hood and noticed that somewhere at the back of the engine oil was dripping on a heat shield. It wasn't a huge amount but seemed steady.

I plan to have it flat bedded to the garage I use to have it checked. It's a 1991 automatic, so reaching out to see if there is a common cause that it would most likely be? I've only owned it for three weeks, but checked carefully before buying and it didn't have any leakage,

 

From your description it sounds like the rear of the intake manifold. When you install the intake manifold there is a piece of rubber that can be used to seal the end of the intake manifold to the engine block. Older Chevrolet vehicles frequently leak from this gasket. My engine builder like many people did not use the rubber piece at all but instead he used RTV to seal it. I have had to remove intake manifolds to re-seal the end of the intake manifold. When they leak, oil comes out and you get a serious vacuum leak as well.

Did you buy the car from a dealer or a private party? If you dealt with a dealer then they might help you, private party and all bets are off. It should have been leaking all the time if the oil leak was indeed from the rear intake manifold seal. You never noticed it before when looking at the car?

Since you are new to this expensive hobby you might want to invest in the Factory Service Manuals for your particular year. Here are a couple tools that could make your life better down the road working on your Corvette. Fuel Pressure gauge with long hose, a Good Volt Ohm Meter and a decent OBD 1 code reader. Good Luck with your "new to you" Corvette.

 

OP, I would *highly advise* to use a corvette, and if possible one that is familiar with the c4, to get them to remove the TPI intake and reseal the engine.

If your usual shop is not a vette specialist then i'd strongly suggest this.

Furthermore, pending your budget, Fuel Injector Connection has a great sale on right now for new injectors which would be a great time to replace them. They are cheap. Lots of other things you could do "while they are in there".

Anyway, if you are not doing the work yourself, and not using a vette specialty shop, you just put yourself at so much risk of lost fasters, stripped parts, etc.
As Closkey says, its likely rear "china wall" between base of intake manifold and top of block. Could also be the oil pressure sensor. Less likely is the valve covers unless its coming out the side a bit. even less likely is the gasket between the cylinder heads and the block.

nothing to be too scared of.

 

Why? This part is SOP. Is it different from any other L98? Didn't the 350s all have that China Wall issue?

 

 

Thank you all for the information provided. It is always great to be a members of specialty forum such as here. So much knowledge from owners that have been through similar problems. I've bought a couple of older Durango's for my two daughter's after they reached driving age so I'm a member of a Durango forum and belong to a Ram Forum for my Hemi 1500.

I did look fairly closely at the Vette and ran it for a extended period looking for any leaks prior to purchasing and no sign of any issues. I also checked several times since buying after getting home from a "firm" driving session and no leaks. It did get a check engine soon light after the extended crank start so they will pull the codes and of course get it on the lift for a good look as to where the leak is coming from. I know the owner of the garage and been using them for 25 years, plus he was a GM mechanic for a dealer prior to opening his business.

I'll update everyone once they've had the opportunity to look at it on Wednesday.

 

Not that bad. I have only had the TPI off twice and installed once. The SR is horrible if you follow the directions. When I bitched to Lingenfelter about the monstrosity he invented, he just laughed and told me there was a trick. You slot the screws under the plenum and can turn it with a screwdriver and do the final tightening with the 12 point wrench. After that, it was easy. Actually, I made it easier. Helicore the runners and screw from the top. Use Blue Locktite to prevent the bolts from turning out.

I don't think I know too many younger mechanics that have done one. Still, I honestly don't think it is horrible. You want to prevent him from cutting corners? Do your homework. Shop BEFORE you need him. Get your oil changed and watch the other techs.

IMO the China is easy to do but hard to do it right. I've seen people use the cork gaskets, dimple both sides of the wall and use RTV or just plain RTV, etc. All kinds of tricks and a prayer to get it to seal without coming out in a year.

 

Isn't the oil pressure sending unit back there by the distributor ? One of you L98 guys should know ….

 

Degrease it all over and dump dye in the oil. Blacklight should show more.

Running codes is easy. Short out the A and B terminals of the ALDL. I don't know if you can gain much if there are no codes. Also with the codes, you have no idea what is going on. The system is very primitive so if the value is plausible, it might not trip a code. I'd invest in a Factory Shop Manual and a scanner (NOT CODE READER which is a glorified paper clip), fuel pressure gauge, compression gauge and IR thermometer among other things.

 

The permatex right stuff won't leak ever. You can also probably lift the block by the intake without bolts after it dries... some of the nastiest stuff I've ever dealt with. I'd bet it never leaks. And if it does it may be way after every other gasket does.


Anyway, to the op, feel the back of the heads for a valve cover gasket issue. Chances are it'll only run down. Oil pressure sender is by the drivers side head, if it is leaking back there it could also spray. The china rail will be more of a seep and shouldn't really leak just idling. There isn't a direct path but rpms and flow will spray it at it.

 

The oil pressure switches are mounted over the bell housing at the rear of the intake right near the firewall. It is very possible that one of these switches just gave up and started leaking, I have had one fail before like that. If it is the switch causing the problem it should be easy as they leak like crazy WHEN they fail. If that was your leak I can see why you would have it towed over to the mechanics as it would be a sizable leak but easy to spot if you know where to look.
Best Regards

 

I have used a mirror with very strong light to inspect the back of the engine for oil leak source(s). It was necessary to first clean the area with solvent and then look for fresh leaking.

Indeed the rear china wall and the oil pressure switches area are good candidates for leak sources.

When I had the china wall leak issue I roughened both the wall and the intake base underside with #30 grit as well as dimpling the wall with a punch. Then I used two runs of black RTV on the wall to assure complete coverage.
Then there is the matter of properly sealing the new intake gaskets and torqueing the base properly.

This is the kind of job it really pays to do yourself since it is more labor intensive than parts pricewise.

 

------ UPDATE ------

So the results are in and those that said oil pressure sensor wins the prize !!!!!! As I said previously, the owner of the garage had many years of GM experience so the oil pressure sensor was the first thing checked. They actually had a hard time finding the replacement part locally, but was finally able to get a hold of it. Although the Carfax report showed the oil was changed fairly recently I'm having them do an oil and filter change before I come pick it up. Thanks again for everyone's thoughts and suggestions.

 

Im shocked.

 

 

Lol …

 

=== Final Chapter ===
Finally had time to pick up car today. Replacement of the oil pressure switch and an oil and filter change came to $135.07. Next project will most likely be to replace the bad Bose amps in the front and rear drivers side in addition to my ongoing work to silence some interior rattles.