93 LT1 only fires when throttle is pushed

[Gene D]

93 LT1 only fires when throttle is pushed. Cranks with no start if pedal is not pushed.
I have to hold throttle about 1/4 way to keep at 1200 RPM.
I have looked for vac leak. Fuel pressure is at 41 psi.
I pulled and plugged all vac ports on the side of intake(the ones under black covers.
I checked all hoses from the air pumps on drivers side. (I think)
No check engine light.
If I put a vac gauge on one of the small ports on pass. side intake, What reading should I get while cranking?
Any other Ideas what could cause this?

 

[Gene D]

The next clue for needing to push throttle to start

I checked compression, spark, I did leak down bump test on injectors. I took opti spark off and went over that.
I wanted to bleed the air out of coolant, so I held throttle down to start, and kept it running for a few minutes.
I noticed both exhaust manifolds were glowing orange after about 5 minutes of idle. They were an even orange on all
exhaust legs. I guess all cylinders are firing.
I think my next step will be to remove the crossover cat and fire her up.

 

[gmjunkie]

Check your injectors,My guess they are leaking down and flooding it!
When you press the peddle down not running the ECM shuts down the fuel and lets it clear out and start!
Also could be a leaking diaphragm in FPR! (Fuel Pressure Regulator)

 

[Gene D]

Fuel pressure holds

Fuel pressure holds at 45psi. The bump test I did I hooked fuel pressure gauge to rail. Brought pressure up to 45 with key. I then touched leads from battery to injector terminals. It took about one second for the pressure to drop to 35. I did this to all 8 injectors.
The pressure will hold 45psi for 10 minutes. Only drops to 40 after 2 hours.
After starting the car,(pedal to floor) I can feather the throttle and keep it running. I can not get it to go above 1500 RPM. After the engine warms up(about 2 minutes), it will idle fine.
It runs smooth at idle, It runs the temp gauge up quick. You can watch gauge move quickly and feel the heat from engine compartment.
After about 5 minutes of idle, It still will not rev past 1500rpm, and exhaust manifolds are glowing orange.
I used my timing light to check spark to all wires with the inductive pickup(checks for fouled plug also). I also have an 8 stud spark checker.
I did the injector Ohm test.Book says reading should be .8 All of my readings are .02. I might have wrong scale on meter. Does not seem right, but all are identical readings. All injectors sound like they hit good when voltage is applied to terminals.(IMHO)
Holding pedal to floor shuts down fuel to clear flood, interesting...
What does a leaking fuel pressure regulator diaphragm do? Will not allow pressure to build, or vac leak?

 

[Robertwav1]

I had a bad fuel pressure reg and it was always hard to start after the car was warm. Cold starts were no problem. This is the way mine acted. I never had an issue with it running though, just hard starts? If you suspect the FPR you would be able to smell fuel in your vac lines.

 

[Hib Halverson]

Your description of fuel pressure testing is not easy to understand but your posts seem to indicate that when you ran the Service Manual fuel pressure test, it was between 41 and 47 psi which is what it should be.

Powering the injectors tells you nothing more than fuel flows though them.

Something is bad wrong with that engine if, when it will run, the exhaust manifolds are glowing, especially since the manifolds are cast iron. That's a s**tload of heat. That's usually a function of excessively lean air/fuel ratio, way retarded spark or some other significant problem.

If it were me, I'd be hesitant to run the engine for very long with that problem occuring.

When you say "remove the "crossover cat" what do you mean?

Describe any recent work/modifications done to the engine...
Camshaft change, maybe?

What's the exhaust like when the engine runs? Black smoke? White smoke? No smoke?

 

[DonB]

Did this problem start suddenly? What's the mileage? Like Hib said, if those exhaust manifolds hot enough to be glowing, there are some serious ENGINE DAMAGING" issues happening. Engine should be pulling about 18 inches of vacuum at idle. Have you checked your timing?

 

[Gene D]

FPR is leaking

I smelled fuel in vac line to FPR. I hooked up my brake bleed vac pump with the catch canister to FPR. I pumped it to 17 vac. After about 5 minutes... there was liquid gas in catch canister.
I have not done any work to the engine. I did brake booster and master cylinder.
There is an oil leak on front of engine. When it started running bad, I wanted to check/replace wires and front engine seals.
I took out optispark and replaced front seals to opti and water pump drive. I checked and cleaned and resealed opti. I greased bearing on opti.
The opti looked good on inside. I did front project after the engine started running bad.
Yes the injector test I did told me all injectors are working. I timed the pressure drop, so they all work the same(good or bad).

Today I removed the drivers side engine mount crossmember. I dropped the pre cat from the exhaust manifold.
I started the engine. It fired up without pressing pedal, and I was able to rev to 3000 rpm.
I did not run long, just a quick fire up.
Removing the pre cat and letting exhaust open seemed to really help. I will look more tomorrow.
As far as the crossover cat. I am calling the front little cats pre cats, and the bigger one further back that joins the exhaust, then separates the exhaust again a crossover cat?
Am I out to lunch on this?
Can a plugged exhaust cause exhaust manifolds to glow red in 5 minutes of idling.
I ordered a scan cable and software. I have not gotten them yet.
You guys were right on with the FPR. Any advice as to where to buy a good one?
Any advice on what might be plugged or what is normal with doing the exhaust drop?

 

[DonB]

A plugged Cat will definitely cause excessive heat and a poorly running engine. The exhaust is as vital a part of the combustion cycle as the intake. Just think what would happen if you could NOT exhale. I replaced my FPR with original AC/Delco.

 

[Gene D]

I have a great idea about this...

Great Idea... look into the tailpipe from the back. I can see a honeycomb piece blocking the tail pieces where the muffler ends and the tip splits.
Pic is looking into one tip with a light shinning into the other.
I don't know if this is from muffler? Or from something else.

 

[DonB]

If I remember the honeycomb was part of the element inside the Cat. Either way it is a restriction that must be removed.

Here in Illinois they don't smog check cars after they are 20 years old, so I eliminated my Cats and the Y-pipe. Each side runs from the vertical section that connects to the exhaust manifold down and along each side of the trans., back to the mufflers. You'll have to plug the smog air pipes that go to the Cat also. No biggie. I gained some power and it sounds wicked!

Disclaimer: I AM NOT ADVOCATING THE REMOVAL OF A GOOD EMISSION CONTROL DEVICE. Should new Cats NOT be readily available, this is your option, depending on local laws.

 

[Gene D]

Honeycomb in drivers side pre cat

Honeycomb in drivers side pre cat is broken loose. I broke honeycomb into pieces and dumped them.
I cant see into pass side pre cat yet. The pipe is longer. At least the drivers side was still in pre cat, and not in the muffler.
I did not get to crossover cat yet.
I did a timing light test. I marked the crank pulley for TDC #1. I am only cranking, not starting... but timing looks like about 15 degrees after top center??
Is this normal to be this late before engine starts?

 

[DonB]

Timing always should be BEFORE TOP DEAD CENTER. The spec varies by engine and model. But it is usually between 6 and 10 degrees before. 15* after is quite a bit retarded and WILL cause poor performance and excessive heat. Could be what cooked your Cats.
You never answered my question, "How may miles on this engine and has it ever been severely overheated? Did this issue start suddenly?
At TDC of #1 cylinder on timing marks on crank. The rotor MUST be pointing to the #1 wire location. Either you have it timed wrong or I'm thinking you could have a jumped timing chain, causing the problems.

 

[Gene D]

120,000 on car. I just purchased it. There are no signs it was severely overheated.
This has an opti spark. It only went into cam drive in one spot.(I think)
Compressions on my gauge are 192 to 195 on all 8 cylinders.

It was running good for a few weeks. It started to miss and stumble a little when it was cold. Ran good when got warm.
That got a little worse.
I took off water pump, coil, and opti.
New coil put on. opti was cleaned, resealed and put back on. Water pump and opti seals were replaced.
I checked the plug wires.
While it was apart. I did a backlash check on timing chain. It was 2.4 degrees of backlash from crank pulley to water pump drive.
It was after this the exhaust started glowing.
The engine has only been ran a few minutes like this.
Can an opti be put in wrong?

 

[DonB]

At this point I'd clear up the exhaust to be sure it flows and get your base timing to spec. Then we'll go from there. There are just too many variables involved to guess.

 

[Gene D]

Way too many things going on

I have a blocked exhaust that I am digging at. It is all welded.
I have an ignition timing issue that I am not sure how to change.
My injectors all ohm at .02. All injectors do shoot fuel. I am not sure about these readings.
I watched some videos about the opti spark install. All the opti's I saw had a three pocket hub that was about 1 and 1/2 inch diameter.
The bigger pocket fit on the camshaft pin.
My opti spark does not look like this. Mine has a 1/2 inch diameter splined hub . It only went into cam when I had it in the exact right spot.
Did I pull opti spark out wrong?
I don't think the timing chain is sloppy enough to have jumped. I am still looking for a good way to check this without pulling chain cover.
I am working on at least getting the (empty) pre cats back on, so I have the O2 sensors in place. Then I am gonna try to fire it up and recheck timing.
I know the timing is pulled back by computer for starting. I just dont know how far this LT1 pulls it back. I would think the timing would be pushed to 35 degrees BTC at high RPM. I don't think the cap and rotor is capable of that much swing.(+15 to -35).
Any good ideas on how to check camshaft timing?

 

[DonB]

As far as I know the opti only fits one way. As far as checking cam timing, the indicator, without pulling the T.C. cover and visually lining up the dots on the gears, is the distributor/opti's position. But now that is in question too. I know it sucks pulling the cover with the engine down on the mounts because of the frame rail right in front of the harmonic balancer. There is no room to pull the balancer off(at least on my '90.) You'd have to get the engine up off the mounts. One consolation is that while the cover is off you could put a new timing chain and gear(s) in and be done with it and be definitely sure it's right. To be honest, in my experience, 120,000 miles is just about(past) the end of a timing chains service life.

Maybe reach out to ecss. He's a knowledgeable guy and he's just up the road from you. He did offer to help you out. Maybe he could steer you in the right direction. Wish I could help you more. Keep me posted.

 

[Gene D]

Timing is off 15 degrees

I fired her up today. The timing jumped to 0 degrees at 650 RPM.
So its 15 after top center while cranking, and 0 at 650.
Seems 15 degrees off both times?
I would expect 0 cranking and 15 BTC after starting?
Guess I might as well drop oil pan and put new oil pump in since the pan needs dropped to do timing cover.
I looked into the passenger side pre cat with a mirror. The honey comb is still in place on that one.
I guess the honey comb pieces I dug out of muffler are from main cat/crossover cat/ spider cat... whatever it is called.
I tapped the main cat, shook it around. Cant hear anything rattling.
It looks like GM spent a million to make sure NOBODY could get the timing wrong on an LT1, but somehow I did.
Are the Axcell opti sparks for $150 any good?

 

[DonB]

Gene I have no experience with the Accel Opti's. I've used their ignition products in the past and they are top-notch. Holley now owns Accel, so don't freak out. It's all good. Swapping out the oil pump is a good idea too. The standard Mellings should do well for you. Make sure the oil pump drive is in good shape too. If in doubt, change it with a Mellings hardened one. They are only a few bucks as they are a known "Achilles's Heel."
It might not hurt to pick up a Factory Service Manual. You can get them on disk from Corvette Central .com for about $30.00. They are a life-saver and explain correct diagnostic procedures. It comes with the electrical supplement too. Where ever you buy it be sure to get the "Red Cover" issue.

 

[DaveP85C4]

I Highly doubt there's anything wrong with the timing chain. Ignore it for now. That's an incredible amount of work for something that is probably just fine.

Ignition timing; I'm not sure. I just read up on it. It does not have a by-pass mode like earlier ignition systems. Timing is totally controlled by the PCM. I did not see any mention of checking or adjusting base timing. I'd ignore timing issues until the other things are addressed.

Plugged exhaust results in very low manifold vacuum. The PCM will interpret the low MAP as a high-load condition, and adjust timing accordingly. This along with fuel from the leaking regulator may be responsible for the glowing exhaust manifold. OR, the leaking regulator caused an over-rich condition that destroyed the cats. Difficult to know. All you can do is fix what you know is wrong.

Your injector test made sense to me. If all 8 have the same pressure drop for the same duration of powered-on, and the pressure holds, they're working, not leaking, and all flowing about the same. I use this test frequently. It will identify a single injector that is not flowing like the rest of them. I question the Ohms reading. ".020" doesn't make sense. 20 Ohms would. That they are all the same leads me to believe your injectors are fine. The 93-up injectors aren't subject to the ethanol problems that the 89-92 are.

Fix the leaking fuel pressure regulator. No-brainer.

Take the exhaust system off and get the chunks out of everything. The piece in the muffler indicates the main (you call it crossover) cat came apart too. You have options as to what it takes to fix it depending on whether you have emissions inspections. I won't elaborate....

Once the fuel and exhaust system is fixed, re-evaluate the ignition timing. It may be fine.