Tbi vs lt1 intake manifold question

Hi all,

I'm trying to decide if I want to take the intake manifold off the TBI to fix a small coolant leak up at the front seal somewhere. It really is quite small, but it's there. The rear heater hose just started leaking too so there's already some reason to drain and flush the coolant.

I did an LT1 intake last summer, how does the TBI compare? Distributor needs to come off, and the ac and alt brackets? I guess it is a "wet" intake too. Any other main differences? Replace all of the tbi seals while I'm at it?

Thoughts?

Thanks.

Aside from what you mentioned the procedure is the same. It is a wet intake, so when you pull it make sure you don't tip or you'll be draining coolant out of the oil.

Maybe I'm not catching on to the significance on your question .

My understanding of it, is since the TBI motor  and the LT1 are not the same block or heads, I would think you would have issues that they just don't match up in terms of bolt holes, head angles and ports.  MY 93 TBI has a conventional Distributor driven from the rear of the cam. Does not the LT1 have an Optispark off the front?

You can get better manifolds for the TBI, but my impression has always been that they are not interchangeable.

lakeffect wrote:

Maybe I'm not catching on to the significance on your question

That's fair. I was looking at the manual last night and thinking back to my experience with the LT-1 and wondering,"do I really want to do this again?"

I've actually never pulled a distributor, and then it seems there are always a list of 13 different irreplaceable items that get broken somehow along the way. At least, that was me with the lt1. Fuel line (my fault), throttle cable (not my fault), one of the sensors etc etc etc.

So I was fishing for insight. Thanks for your thoughts on it.

I have pulled a number of the TBI units. They are really straight forward, just like a carb intake. You have to pull the TBI, coil, cable bracket, and the EGR solenoid bracket, to get to the bolts. Mark your distributor's position before you pull it. It will turn about 20 degrees when you pull it out. They are cheap enough at the junk yard, that it is easier to just replace it than to try to fix it. There is only one aftermarket TBI intake available, and it looks just like the OEM unit. When installing it clean the surfaces and dry them, and put plenty of silicone sealer along the front and rear, or it will leak. The gasket kit will have some instructions to help you. You really need a helper to place it correctly the first time you try. If you have to adjust it more than about 1/16 of an inch, you will have to put new silicone on the front and back. The rear bolt on the driver's side is difficult to get to, because the bracket there blocks your access. Do not over torque the bolts. Make sure you keep the bolts in the same place, because some are longer than others. Wet intake refers to the fuel going through the intake, not water in the intake. You will not spill any water if you lift it straight out. There is only about an ounce or two of water in the intake after you drain the system. The fuel lines will get in the way as well. Make sure you have everything out of the way when installing, so you get a clean shot at locating it perfectly the first time.

Great insight, thanks Fred. I ordered up the parts so I will see what I can get done...

-Leland

You are likely to be already familiar with the fact that there is a specific sequence to tightening both head bolts and manifold bolts, and a torque in ft/lbs. Most rebuild manuals will have it for you.

Yeah, hopefully I can get that right. It would be nice to only do this once AND not make things worse in the process. I've got a SM to help along the way too.

Send me your email, and I will send you a picture of the FSM tightening sequence.

Got started on this today, hopefully finish it tomorrow. So far only one irreplaceable part broken (vacuum fitting that should be easy to work around).

I can see that as Fred said, the fuel lines are interesting to deal with. I hadn't looked closely at them before and was expecting them to run up the back side like on my old suburban TBI.

While you have it apart, and if you have access to a 91-93 FW in a junk yard, they have fuel lines with Schrader valve taps on them, so you can check your fuel pressure.

That would be a good idea. I haven't run across those unfortunately. Right now I would just be happy to get the heater hose off the firewall and the distributor cap 'unwelded' from the small screw that hold it down. Good grief.

There is a small screw in the back like the one in the front of the distributor. It seems like it takes about 300 turns to get it off. I use a 5.5mm deep socket, and a short extension for my quarter inch drive ratchet. Once you get it loose a couple of turns, you can use the socket and extension like a nut driver. If you keep turning the screw, it will finally come off of the distributor base.

Unless you recently replaced the cap and rotor, I would get new ones. To remove the rotor, do it before removing the distributor, or after reinstalling it. I prefer before. You usually have to pry it off with a large screw driver, but do not use the distributor as a pivot. Place a block on the manifold and use a long screw driver. It will probably fly around the engine compartment when it comes off. The shaft is usually rusty, and you should use some Scotchbrite to make it look nice before installing the new rotor. You will probably have to use some force to get it seated, but it will only align one way, because the shaft has a keyway, and the rotor has a key molded into it. The distributor may not come out with the rotor on it, and that is why I remove it before removing the distributor. I carefully tap it back on using something to avoid contact with the center contact (small stick on either side?). Since you are in such close quarters, a heavy hammer with short strokes will seat it.

I did not remove the hoses when I did my manifold. There is a fitting on the top rear of the manifold that has a hose to it, and that hose can be turned/held out of the way. If you insist on removing the hoses to the heater core, the hoses are held in place by white nylon retainers. You have to press both sides at once to get it to release. The white nylon part stays with the heater core. You do have to be a contortionist to get the hoses off.

The firing order is cast into the manifold in front of the TBI. Mark your ignition wires before removing them from the cap. If you are changing them, do them one at a time. If you are replacing the plugs, the two in the back can be easily reached from the bottom (do them individually with each wire). I have small hands, and can get the rear ones from the top, but it is a lot easier from the bottom.

Thanks Fred,

Well ... It continues to be a work in progress. I got the distributor off, and would have left the heater hose alone except that it was already leaking and needing to be replaced so I had to deal with it anyway. The new one seems to have come with the nylon clip so it all gets to be done afresh I guess.

The intake came off, and left about 2/3 of the gasket welded to the block. I finally got that cleaned up and the mating surfaces on the intake as well, then quit for the night. I'll get back to it tomorrow.

I will be somewhat shocked if I get it all back together and it runs. The distributor was a little bit of a pain coming off, though I bet the next time it will be a snap. It makes me concerned about alignment going back in though I did mark it.

Anybody think it worth replacing the EGR valve right now since I'm starting at it and the engine has about 210k on it? As far as I know it's original.

Hi everyone,

I got things buttoned back down and all of the fluids back in. That was after at least one miracle of not dropping a screwdriver down the distributor shaft as I reset the oil pump fitting that had moved when the distributor came out.

It is having a very rough idle for the first few seconds of being on, at which point it seems to go into some kind of preset operating condition, turns on the CEL, and runs okay. I've reset the timing by unplugging the two wire connector atop the passenger side valve cover but with no notable change. Any thoughts on what I'm missing here? The CEL does not actually set a real code according to my reader.

I'm wondering about the IAC. Maybe a failed connector from all of my activities of the past few weeks?

Thanks for any thoughts,
-Leland

Likely went into"limp home" mode. What did you set the initial base timing at , once you unplugged the brown connector wire? Instead od being something like 6-8 degrees BTDC, should be about zero.

You'll need to read the codes to find out what the malfunction is. There is a procedure for setting the AIC and the TPS sensor. TPS should be right about 0.54v with ignition on, engine off.

I checked a TBI that I have, and the new style TPS is not adjustable on the TBIs. The IAC sets itself, and you probably did not remove it either, so it was where it started, and should be fine. The base timing is set at "0" only. Did you make sure you can read the timing marks before you used the timing light to check it? Check your ignition wires again to make sure you have them on correctly. Sometimes a couple can be crossed, and still idle, but not happily. Did you check for vacuum leaks with a squeeze bottle and some water with a couple drops of dishwashing fluid?

The gaskets usually affix themselves at least partially to one surface or another. You have to make sure you do not get ANY of the parts into the intakes in the heads. If you did, it can be a little iffy when they try to go through the combustion chamber. They probably won't break anything, but can hold a valve open a little if they get trapped under it. I assume you can figure out how the engine would run if that happened. Once the particle has passed through the combustion chamber, the engine should return to normal running. Something like that could throw an O2 code if it was an exhaust valve that hung open.

Try taking the fuse out of the small holder at the antenna and fuel pump relays for about 60-120 seconds, then put it back in. That will erase any codes in the computer without disrupting your radio presets, and AC settings.

As long as you have it timed correctly, all the ignition wires are correctly connected, no vacuum leaks, and you plugged everything back in, you should have a good running engine. The computer will figure it all out.

I would not worry about the EGR valve unless you have an associated issue, such as not idling/stalling. I have replaced a couple, and the ones I took off are still good. I was using the swap parts style of troubleshooting. If you want a good EGR valve I have a couple for the price of shipping.

Thanks all, it's too bad the ECM isn't recording a real fault. That might help but it just flashes the CEL once the limp home mode is on (I didn't realize obd1 had that).

I'll double check a few things, of course. And the timing is set now to about 0-1. It seems like most were happy with it somewhere in that 0-2 range, 3 being a bit extreme.

I'll let you know if I turn anything of interest up.

I would check the vacuum line that comes off the back of the tbi and goes to a solenoid on the passenger side firewall. Make sure the line isn't cracked. It will cause a surge idle condition if there is a vacuum leak to it. I noticed you said you aren't getting a code from your reader, have you tried just jumping the adl connector to get the CEL code? There's also a diagnostic mode you can uses that should tell you if the car is running in closed/open loop. I posted it as a reply to a previous thread a while back. I'll search for the thread and post the link.

Does anyone else have problems getting the search function to actually find older posts?

found it  
It's called:
[You must be registered and logged in to see this link.]
You are out in the middle of nowhere without any tools. Here's an easy test that can be performed on any General Motors fuel-injected vehicle for 1982 until 1995(1992 on some models). All you need is a jumper wire or paper clip! Whether the engine is in closed loop or open loop, a rich or lean condition can easily be determined by connecting terminals A and B in the 12-pin DLC and starting the engine. (This test is not available on vehicles equipped with the 16-pin OBD ll connector.) Check the operation of the "check engine" lamp (MIL). With the engine running and the diagnostic terminal B grounded (DLC terminal A connected to B), the "check engine" lamp should be off when the exhaust is lean and on when it is rich. The procedure is called the field service mode.
l. Open loop. "Check engine" lamp flashes at a rapid rate of two times per second.
2. Closed loop. "Check engine" lamp flashes at a slower rate of one time per second.
3, Lean exhaust "Check engine" lamp is out all or most of the time.
4. Rich exhaust "Check engine" lamp is on all or most of the time

This might help in determining what might be causing the rough idle. You say after a few minutes it clears up I'm guessing because ECU compensates and goes into one of these modes to control the weird idle/condition.

Hope it helps.

Thanks for that. I'll check it all out and let you all know.

I'm leaning toward a closed loop operating mode after about 5 sec of rough idle each start, says sensor to me but I'll point a little carb cleaner at things too - and thanks for reminding me of that little tube out the tb backside. I'm happy to be wrong about a sensor if it cleans up the running.

Spent some time with her and found that small little vacuum tube out the backside was leaking....

Because someone forgot to hook it up.   

One possible fault code to double check tomorrow, but she is doing a lot better. I even gave her a bath.

Thanks for all of the help along the way everyone.
-Leland

That small tube goes to the MAP sensor, and will cause the engine to run poorly.  If it is not hooked up the computer will substitute a "standard" value for the part, and remain in open loop.

Thanks Fred, the mystery is solved then.

I appreciate all of your help along the way.

Yeah I had the same idle problem when I did some work on Ms. Roadie a while back. Found out just how brittle those little plastic vacuum lines are when I barely touched mine to hook it back up and it snapped in two. I thought no big deal I'll start her up any way. She was surging like an alcoholic downing a fifth of tequila. . .gulp gulp gulp. . .breathe. . .gulp gulp gulp.