Well, got the call from the mechanic, all the diagnostics were fine, no codes. He says he lots of experience working on vettes and a common problem is the intake manifold has a possible crack. The car is only pulling 6 lbs of vacuum. Recommended taking it to a preformance place to do the work How hard is is to pull the TB, Plenum, Tube section on this L98? He said if cracked it should be easy to spot once all is removed. Im willing to jump on this if you guys think it is possible. Any suggestions ?

 

Did they check the timing?

Seriously retarded timing could result in low vacuum readings.

I do have 2 intake bases lying around if it turns out that you need one.

I can't say I've heard of these intakes cracking before, but I guess it is possible. Of course, there are many possible places for vacuum leaks on the L98, with all the mating surfaces for the runners and plenum.

 

Intake is no big problem to fix if thats the problem. We had a local shop fix the one on the Big Block a couple of times ( could never tell it was fixed when he got done)when my untill my buddy got use to the torque wrench. But there are several other things to check as desribed above.
Good Luck wro87

 

check all of your vacuum powered units and hoses before some yayhoo tells you the manifold is bad.
sounds like he just wants the labor to change manifold gaskets. I heard that can be upwards of $600 and nothing is really accomplished except he finds the vacuum leak, and you get a new set of gaskets.
take the car home, and do the following:
1. start the car, and use a set of pliers wrapped in making tape, and squeeze the vacuum hoses one by one, and listen for the engine sound to change momentarily.
2. make sure the vacuum heater controllers aren't leaking
3. get a syringe and fill it with gas, and put a little on the fuel injectors where they go into the manifold, the "o"-rings can be leaking.
4. squeeze the brake booster vacuum line, and see if that's the problem.
5. check to see that the throttle body bolts are good and snug.
6. get a torks bit, and check that the manifold button -head bolts are all tight.
7. check to see the pcv valve hose is not split at the rocker cover or on the rail where it goes into the manifold.
8. check the little check valve for the cruise control at the firewall next to the left side of the distributor behind the left rocker cover
check check check check
sometimes the mechanic is pushed for time, and the first thing he hears or sees is the problem, and it may not even relate to your car at all.
If you have low vacuum, get the guy to do a compression check. youo may have crud on the intake valves that's allowing them to not seal properly.
check check check check cause it's your money! not his.

 

*Edit: I stand corrected!

 

I agree 100%. Get the car back and either get a second opinion, or do some diagnostics yourself.

Does the car run hot? What are the LT and ST Fuel Trim numbers?

Though a compression test or a cylinder leak down test is a good way to tell if the engine really is weak, it won't tell you if you have a vac. leak in the induction system. That you'll have to find the old fashioned way.

I have seen L98s leak through the plenum gaskets or a worn out throttle shaft bushings, but I don't think we have run across a cracked intake manifold before...but never say never, I'm sure these guys have been around alot longer than I and have seen stranger things...

Pulling off the intake manifold is not that difficult a task, its just time consuming. Just take your time and don't forget, that not all the plenum runner bolts are the same size. Mark them if you can. Don't torque down any of the runner bolts until you have ALL of your bolts started into the plenum.

Good luck.

 

They do crack.
The motor I just pulled had one on the PS near the coolant jacket..Had that welded up awhile back. leaked coolant, not vacuum.

He then got carried away with wanting to clean everything so he pulled the tin cover off the bottom, and VOILA! Big old crack smack middle in the bottom of it.

Lucky strike.

 

Again, I am proven wrong. Good find!

 

At a cold idle it runs fine. Acceleration results in slight vibration. Once it warms up, setting or waiting on a traffic light, a faint knock occurs. The longer it sets the louder the knock gets. It starts out knocking approx. every 8-10 seconds, if you don't rev it up it will knock every 2-3 sec. after 2-3 minutes of idling. It runs fine on the hwy and not too noticable, except for the vibration. Listening underneath it sounds very scary as the motor jerks and jiggles, it can also be felt thru the shift ****. I will try and trace down some vacuum hoses with carb cleaner here shortly. P.S. the mechanic just recomended I take somewhere that specializes in that area, after a long talk on the phone he gave lots of good info. Once he relized I would attempt it myself he was very understanding and helpful. Recommended a compression check as well as a smoke test to find the leak.

 

Never is a looooong time!

 

One more thing, the mechanic did mention a valve might be not seating right, if you have the whole plenum, TB, etc. off, would those have to come off to get to the valves? Ive got the GM service manuals and haven't researched it throughly, just thought I would ask ( no wise cracks please )

 

Again, lucky find. Looks like it started out as a parting line from the casting. Interesting.

 

You mechanic is used to the new engine technology in use on todays engines where the intake manifolds are made out of plastic or composite. I replaced mine with an Acell foir my SR. The are both solid aluminum castings, my old one is about 25 lbs of recycleable aluminum

 

You mechanic is used to the new engine technology in use on todays engines where the intake manifolds are made out of plastic or composite. I replaced mine with an Acell for my SuperRam. The are both solid aluminum castings, my old one is about 25 lbs of recycleble aluminum.

 

Dean: just for sh-ts and grins, check the resistance on you Multec injectors. Spec is 16.5 +/-.3 ohms. This is easy to do and may not be a factor in your problem. However, you can rule this out with a simple test.

 

wel, I read your thread reply, and it knocks at idle? be more specific
1. is it a valve tap?
2. is it a piston slap?
3. is it just a rough running engine that is jumping around on the
mounts?
4. Is the air conditioning off?
5. Is it the air tube for the cat hitting the frame?
knocking at idle is reminiscent of way low oil pressure letting the rods knock.
another thing that will make that thing run like a washing machine filled with rocks is an egr valve not closing all the way. It's hard to accomplish, but I use an old brake spoon that I got for my 57 chevy, and I wedge it under the diaphram disc in the EGR and open the valve and see if the car runs better after that. sometimes krud gets stuck on the EGR seat, and the valve doesn't close all the way. just make sure you don't rip the diaphram. The plenum has to come off to change it.
I had a 70 1/2 camaro that would knock sometimes, and GM sells combustion chamber cleaner. I would use that to clean the excess soot out of the cylinder surfaces and the carbon on the heads gets dissolved. Um on a Fuel injected engine, I dunno, I would probably add it through the pcv valve.
anyway, if it's knocking at idle, you are going to limit your troubleshooting parameters. those things are the first things I'd check.
How old are your plug wires
check the red and the brown wires that plug into your distributor cap for tightness, loose power wires are undetectable by the computer.
take the dist. cap off, and look at the coil button that protrudes into the cap to touch the rotor. make sure it has plenty of spring, and it's not wearing a hole in the rotor tang
check check check check!!!! sorry, but i'm having a good time here lol

 

When the car was giving to me by my stepmom she spent close to 4000.00 at a local dealership to get it running( it sat for years)The injectors, distr. coil, wires were all replaced, tune to spec., flushed all fluids ( the dealer did everything it could to take her money, she just wrote them a check). The car had 16K miles 4 years ago, it has 38K now. Got back in town today and I'm going to start with vacuum hoses, check the EGR, try anything before I start tearing off the Plenum. The knock is noticeable near the throttle body, but underneath the sound is hard to place (will have to ramp it to get underneath and pinpoint)but its like within the oil pan. Its a shame I was off all week and shopped for a van for the wife and never got to work on the car. Will concentrate on the car this week and weekend. All the info from you guys has got me thinking and confident I will solve this problem. Thanks alot and I will stay in touch as I prod along.

 

If you want to check the EGR valve you can simply connect a vac line to it's input while the engine is idling. If you use enough rubber vac tubing you can simply pinch off the tubing to see if it effects the engine vacuum or idle smoothness. You may need a "T" in the line so you can bleed air in to allow the valve to close after you pinch off the vac line from the plenum fitting.

Carefully pry the line off the EGR solenoid and use some rubber tubing to connect it to the plenum port used for the FPR; you can idle the engine without vac to the FPR for this test.

There are a few hard vacuum lines that you will need to simply (and carefully) remove so you can plug the ports to check for leaks.

Connect a timing light in turn to each spark plug wire to see if any show a misfire.

If you don't have a stethoscope, you can use a length of rubber tubing to try and locate the source of "knock".

A comperssion check will show if there are internal problems like a bad valve or leaking piston rings.

Also check oil pressure when you hear the knocking; see if it drops much after warmup.