The Chevy LT1 350.

[piece-it pete]

What's not to like?

I've got a question for the gurus and would-be gurus like myself.

My dad drives a 93 Buick Roadmaster (station wagon). The LT1 was standard on this whale of a vehicle. It's at 116K miles, still runs smooth and strong.

But as I was driving it (piloting it?) yesterday I noticed it hesitates coming off a stop, almost like it's gonna die.

Any idea what's wrong?

Pete

[jpdylon]

could be a clogged fuel filter, worn fuel pump (have fuel pressure checked), bad EGR valve, faulty Mass Air Flow sensor, or throttle position sensor. Even somthing simply like a really dirty air filter can cause this condition. Don't neglect the O2 sensors either, they are probably the biggest cause of poor performance.

If it hasn't been serviced or had a tuneup, nows the time! if you take care of them and use synthetic oils, the iron block LT1s can go for a long time.


Jordan

[piece-it pete]

Thanks Jordan.

Here's the thing: my dad is a maintenance fanatic, so tune up items aren't the issue. He drives most of his cars to around 200k. I think he's running a synthetic oil blend.

Since there's no problems on the road, I can put my foot down and this massive thing goes zoom (awesome cruiser), it wouldn't be the fuel filter, right? Does the fuel pump fall into this catagory too?

I wonder if some of those sensors have been changed . I think I'm going to pass along the whole list. Darn it, I was hoping someone would say, "Oh yeah on the LT1s the ______ goes bad all the time. Just pop a new one in there and you'll be good to go".

While I'm dreaming, I'd like a pair of Avante Guard Trios! Cars. Gotta love/hate them.

Thanks again,

Pete

[scolba]

auto or stick? If its an auto, it could just be that natural hesitation when you really mash on it....most of the autos i have driven without hop up kits on them (note: these are diesels, but the principle is the same) hesitate like that....

[piece-it pete]

scolba,

Thanks for the reply. It is an automatic.

It's definately a problem, although not a particularly bad one (it still runs!).

Pete

[Jstas]

If you have an Idle Air Control valve, if it is dirty or sticking it may cause this problem. Otherwise, everything I could think of was already mentioned.

What the most likely causes are are things that deliver fuel or meter delivered fuel. Things to check that are easy are your fuel filter. That's about it. The other sensors can be tested in most cases but you will need to read up on a service manual to find out the specific procedures.

One thing you didn't mention is if this occurs only from a dead stop or does it occur at speed too. Like if you are crusing along at 55 and want to reach 70 in a hurry, when you stomp the gas, does it stumble then too? If it does, O2 sensors are the most likely culprit with the throttle position sensor a close second. The EGR system would cause that kind of a problem too.

If not and it is just off-idle response then check the fuel filter, fuel pump and fuel pressure regulator. The idle air control valve would only be a problem with off-idle response so that should be checked too if this is the case. I think that transmission is a version of the 4L60E which is electronically controlled. You'd have a service light come on if there was a problem there. The mass air sensor would cause this problem but it would probably also be accompanied by poor running overall. However, if there is a leak after the mass air sensor, it doesn't mean that the sensor is bad but that unmetered air is getting into the system and causing a lean condition. This can result in misfires that seem like stumbling in off-idle response. You'd want to check throttle body gaskets and the intake hoses, even manifold gaskets and vaccum hoses as rare as they are on modern vehicles.

That is where I would start with my limited knowledge. A mechanic would probably be able to give better/more specific advice.

Also, check your spark plugs and wires if it has them. If they are worn or bad, they can cause the same kind of problem.

[Wornears]

At that mileage, and for an off-idle stumble, I'd start by looking at the Throttle Position Sensor (TPS). Easy check and set-up with a VOM. They are fairly robust, but not invulnerable; and can partially fail and not put out their full voltage.

It's not a bad idea to just replace it -- at least that's what one of my wizzened Ford service buddies recommends on every Ford with over 75K miles -- just for insurance and because it is a primary sensor input to the Engine Control Module (ECM). Not too costly, either. Yet our Ford Aerostar 4.0L has 191,000 miles and the original TPS.

LT1s are also notoriously vulnerable at the ignition distributor; to quote one website --

"Also the LT1 has an optical distributor mounted on the front under the new cam driven water pump, as opposed to the tradition belt-driven one. This flaw in design would turn out to haunt all LT1 owners."

Yeah, the engineer that came up with that stellar idea should not be allowed near an ignition system. Check out this site for LT1 diagnostics:

http://www.go-fast.org/z28/diagnostics.html#vibe-low


YMMV. Good luck.

[rek50]

You could start up the "Jewel", and lightly mist the vacuum hoses (Spray bottle/water, or Windex) if any are leaking (sucking air) it will stumble. In the dark, lightly mist the ignition wires, look for a "Blue Corona" indicating the unshielded (bad wire) path of least resistance. If it's fuel injected, maybe the FPR (Fuel Pressure Regulator) is spent. Locate the FPR and a possible vacuum line connected to it. Remove the line and check for GASOLINE. There should be NONE. If there is, the FPR diaphram is no longer a "Cherry". If it has a carb, check for a weak accelerator pump "Squirt"- Engine not running, look down the primary side (by choke blade), while moving the throttle. It should squirt as SOON as the throttle is moved.

[jerrymrc]

At that mileage, and for an off-idle stumble, I'd start by looking at the Throttle Position Sensor (TPS).

Thank you for saying that. They go bad on GM,s more than they should.

[MCS Guy]

I have seen no one has mention this. Get some throttle body cleaner and scrub the thottle plate and bore with a rag. I have seen GM's with excessive carbon build up causing stumbling and sticky throttle.

Also, check you fuel pressure. 30 psi is the minimum for these engines.

[piece-it pete]

Thanks everyone for the great help!

I have passed all this info to my dad who will discuss it with his mechanic, when it's fixed properly I will post the results.

As I drove it more I became convinced it's an engine control problem, vs air fuel or ignition, know what I mean?

And one thing's for sure - this hulking monster of a gm is a lot more car than my 97 Grand Marquis! I LOVE the LT1.

Pete

[piece-it pete]

Success!

Replaced the EGR and the O2 sensors - it now runs GREAT.

Thanks for all the help!

Pete

[piece-it pete]

Resurrect a dead thread....

His gas milage dropped like a stone lately, 17 mpg to 12-13 .

After his mechanic financed his kids' college looking for the problem, he finally found that the intake manifold was loose!

So it was messing up the air/fuel mixture. Now, back to normal.

Just fyi.

I still love that car.

Pete

[MitsuMan]

This is where the real LT-1 used to live................70'-72' Corvette.



damn kids....................

[quaddriver]

Originally Posted by piece-it pete

he finally found that the intake manifold was loose!

Pete

heckuva thing....