Nathan Lord

Can someone give me advice on how to get the l98 plenum off? I've watched multiple videos and believe I've got the runners disconnected from the plenum, but I can't get it loose. I need to change a fuel injector, so that's why I'm taking it off.

confab

L98?

I had to loosen the bottom bolts also to get mine out. There's a couple of bolts kind of hidden on the lower part. Use a good quality torx socket and a swivel on them and they're not too bad. You get the runners loose and it comes right out.

Nathan Lord

that's what I was afraid of. bought the car from a buddy and he said it was in the shop before for injectors, and it looks like some idiot stripped one of the hard to reach lower bolts.

DMITTZ

When I had the TPI intake I could get the plenum on and off without lossening rhe runners.

Don't forget to disconnect the throttle body from the plenum, the Throttle cable bracket, the power brake booster hard vacume line and the MAP sensor on the underside.

If all of that is disconnected you should be able to get the plenum out. Try grabbing it with both hands and rock it upwards it should move, I would think.

confab

Yeah, that is a problem. Use a real good quality torx socket because if you strip the head, you're going to have to drill it out.

If you strip a bolt in the intake, it's really not a big deal heli-coil it and it will be fine.

I have a couple of mine like that now. Quick fix. They work perfect.

Don't overtighten. Torque the thing together as an assembly. Snugging all the bolts slowly to pull it back together.

confab

PS: If it uses the old Multec (SP?) injectors, those things are pure junk. Trash them all and replace with Bosh III from Fuel Injector Connection.

I swear I think 90% of the board have his injectors in their cars. Everyone loves them.

corvetteronw

Your gaskets between the plenum and runners are probably stuck on after all these years.
Make sure you have ALL bolts removed between the intake plenum and the runners.
Make sure the vacuum tube is off (I think it goes to the power brake booster).
Tap on the plenum with a rubber hammer or a regular hammer and a block of wood. Don't try to be the circus strong man but give it a good solid whack.
If that does not move it then try loosening up the lower plenum runner bolts from the intake manifold.
If that does not work then get back to us.
For your own sake go ahead and buy a full set of Bosch III injectors from Fuel Injection Connection. You don't want to do this job more than you have to.
Buy a new gasket set for the runners and buy new O rings for the connections between the fuel lines and the fuel rails.
If you really want to be thorough then also replace the gaskets between the intake manifold and the block.
While you are in there you might want to replace the EGR valve, thermostat, fuel pressure regulator rubber and take the fuel rail apart and replace all of its O rings.
This starts to get crazy doesn't it?
This is also a great time to degrease and power wash the block, install a new water pump, flush your cooling system and clean the crud out between the radiator and AC condenser. If you have a factory oil cooling system then it is a great time to install new hoses in the system.
I did all this on my 91 a few years ago. The whole project started with a fuel injector change. Oh yeah, EGR eliminator, repaint of the valve covers, intake system along with new plugs, wires and distributor along with some new sensors. Visa loves me.

corvetteronw

Nathan Lord

does all that need to be disconnected before I try to get it off

Nathan Lord

wish I had both the time and money for that. this is my last summer with the car before she goes away for a few years while I'm in college

DMITTZ

Well its all got to come off anyway... and some of the things like the hard vacume line to the brake booster and all the stuff connected to the throttle body will make it pretty hard to remove the plenum otherwise. It should only take a few min to remove all that stuff. The map sensor connection could wait till you have the plenum partly removed before you unplug it, but be careful not to forget it.

Don't sweet it though this should be like an afternoon job even if you have to loosen the lower runner bolts, but I don't think you will.

Cliff Harris

The tops of the runners are at a slight angle outwards. That makes it easier to get the plenum out. The first time I took mine off it was quite a struggle. I have a vague recollection that I put a crow bar in one of the holes in the front of the plenum.

Nathan Lord

I can get Bosch 3s for 110 with 3yr warranty or Bosch 2s with lifetime warranty for 90. both are manufacture refurbs. opinions?

confab

I have reman III's in mine.

It's like a new car!

The Multec's (sp?) apparently don't hold up well to ethanol in modern gasoline.

lovmy90vette

Wow! this thread was really useful! I have a 1990 coupe that I and starting to replace my original injectors with New Delphi 22lb flow matched injectors from Fuel Injector Connection. Also bought the plenum set of gaskets. My question to all the experts is Do you install the injectors first then attach them to the fuel rail? And is it necessary to disconnect the battery while working on this project? John W

Nathan Lord

From what I know, they go on before the rail goes back in. I didn't disconnect my battery and haven't had any issues. This is the video I've been using to help me.
https://youtu.be/TMCrylOxsOU

Joe C

my 2-cents, FWIW -- I've done several sets of boschIII's on L98's and I've found it easier to install the injectors in the fuel rail FIRST, then lower the rail in place while carefully fitting the injectors into the intake manifold. as I recall, I do one bank, a little at a time - going back and forth between banks. when everything seems to be seated, all that remains is slight downward pressure on the fuel rail. seems to me, this is opposite from FIC's instructions. the edges of the fuel rails where the injectors are inserted have sharp edges, and doing the opposite lessens the chance of O-ring damage. I would think the same would apply to any type of injector.

if I have to disconnect anything under the hood that's electrical, especially if it's related to fuel, I always disconnect the battery. just a habit on my part. it only takes an extra minute, so why take the chance?

Nathan Lord

Finally got the plenum off! but now, how the heck does the fuel rail come off? do I have to take off one of the runners?

slhawkins

I removed the passenger side runner and loosened the driver side when I removed my fuel rail. I think I've seen a post or two about other people who were able to successfully take their fuel rail off without removing the runners, but I like the open space to work with so I didn't even try that.

The fuel rail has 4 bolts holding it down near each corner and two bolts at the fuel line connections towards the front of the motor. With the runners off that's all pretty obvious, so I'm not sure how easy it'll be to see that with them on.

Once the bolts are off, it might take a little prying to get it loose. I was replacing all of my injectors so I just pried on each injector just a little bit until it popped off. I'm extremely careful with fuel components because of the obvious risk should something not be right when it's installed, so be careful when prying! It might also be worth replacing the o-rings in the fuel rail, it's a pretty easy process.

As mentioned by Joe, I believe the FIC instructions say to put the Bosch III's in the manifold first, then carefully place the fuel rail on top. I have done it that way all three times I've had the top end apart and haven't had any problems. You definitely need to be careful though because the fuel rail edges are definitely sharp and can damage the o-rings.

I personally would just take the runners off though. This isn't a job you want to screw up and either burn up the motor or have to do a second time. I might be a bit too cautious, but I'm not a fan of making it more difficult than it has to be. :-)

Nathan Lord

I'm having trouble getting out the one between the back 2 runner pipes. The one the valve cover makes a pain in the a**. Any suggestions? it might actually be stripped from all my attempts but I've thought a lot of these torx bolts were stripped and came out no problem. do I need to get a T40 Allen wrench style to get to it?