I recently bought a 94 LT1. When I bought it, I knew it had a miss, so I replaced the distributor, cap, plugs, plug wires, computer. Now the miss is gone except about every thirty seconds to a minute it will stumble. When going down the road at speed, the valves kind of ping and sometimes sounds like it's backfiring through the intake. I took the old knock sensor from the old computer and placed it in the new one, now I'm at a loss. The distributor and all the rest of the stuff fixed the miss, but revealed this problem now. What now? Fuel injectors, head gasket? Not showing any codes. Thanks.

 

1. Replace fuel filter.
2. Verify proper fuel pressure.
3. Chances are the fuel injectors need a good cleaning.

 

What temperatures is the digital dash when it is happening versus the analog temp gauge (water pump's ECT to pcm to ccm versus passenger side block sensor)?

Could it be pre-ignition due to heads having hot spots? Whereas most complaints where this is the problem are at idle, you never know.

Since the coolant system is a reverse flow (cooling the heads first) try a coolant change, flush, and stock 180 AC/Delco thermostat (per manual remove both knock sensors to drain block, but very few folks do -- one on driver side, one on passenger, only accessible from under car).

Pay particular attention to not dumping coolant on your new opti-spark distributor.

Also, make sure you have good coolant flow through the plastic heater hose 'valve' (I think it is a pressure reducer for the heater core).

Pay particular attention to having flow through the head's air bleed tube to both cylinder heads (metal tube at rear of heads that comes around to the passenger side and connects to the radiator bleed hose at a metal joint and then to the lower of the two top hoses on the surge tank).

Hope this helps...

 

It at all temps.

 

All temps? Back to fuel...
try the above. How many miles are on the car? If the car has lived in a hot climate(or even coming off the summer) the injectors may be dirty and going wacky from the gas deposits gunking up the pintles. This is by no means an endorsement to simply send them off for cleaning right away. have the flow checked before dropping the 100 bucks for cleaning. Are you running full 91+ octane? I'm not one to blame bad gas for anything, but you could have a clogged filter as mentioned above.

 

O2 sensors

 

what prompted you to replace the computer? you mentioned you had replaced the dist and cap, being 94 model i am assuming you mean the opti. did you replace just the cap or did you install a new opti? one problem fixed and another shows itself has me leaning on ignition or there about issues. backfiring in the intake ?

 

What kind of mileage? If you are getting backfire through the intake it sounds like the timing is off. It's a pain to check but my guess is there's too much slack in the timing chain. My 93 was shot after 75k but your mileage may vary. A popular upgrade is to use an electric waterpump and then switch to a true cloyes double roller setup.

 

I changed the whole opti. I don't know how it came about the the computer got changed, just one thing led to another... two sets of plugs and plug wires and one bad opti from a local parts store and then a new one from the dealer and the major miss that it had is gone, now it just acts like a fuel injector . . . I hope. Do you think that if one cylinder wasn't firing for a while (the #3), that it could have damaged the fuel injector on that cylinder? I'm gonna pull the rail this weekend - if it's not in there, I'm at a loss.

 

I think you need to apply the kiss principal.
Replace the knock sensor with a LT4 knock sensor and see if that helps first,about $29.00.
If your milage is low around 15 mph on a long 20 min drive and no stop lights ,I would try replacing 2 of the 3 o2 sensors(front).
Is the trans shifting correctly?
I would make sure all the weather pack connectors are clean and have dilectric grease and are secure.
You said something about valve ping?What do you mean by that.
If you have noisey lifters you will get a stumble IE knock sensor is retarding and advancing the dist and does not know where to keep the dist so it is advancing and retarding to much.
You have a 94 ,the computer in your car is not only programmable but you can set the computer up yourself.Usually the new computer needs to flashed .94 are almost the same as later modles but you have an ob1 and the later years have ob2.
You have just about covered everything on replacement list.
You have a non adjustable throttle possesion sensor a map sensor on the intake manifold and a throttle body that may just need a cleaning.
Check the fuel presure at the shradder valve it should read 43.5 psi.
Make sure the plug wires are not caught on the drivers side under the A/C compressor and arching.If the engine pings hot or cold I don't think it is a temp problem unles it is running hot over 220 degrees.
Need more specific information as to when it happends.
Hot /cold
High speed.
Low speed.
Trans shifting ok?
Full throttle /light throttle?
Up a hill /flat land?
A/C on A/C off?
Does engine idle smoth in gear hot /Cold ?
Any vaccum leaks?
Does cruise control work.
If you can answer these ?
The corvett form can most likely fix the problem ,with this information.

 

I think you need to stop throwing parts at this problem and actually diagnose it. Unless you like spending lots of money...

The LT4 knock sensor is completely different than previous years. It has a different resistance and is not compatible. I think REDC4CORVETTE is referring to the "LT4 knock module" This is a chip that is replaced in the ECM and is less finicky to valvetrain noise. I would only do this if you have upgraded to roller rockers. Is your vette mostly stock?

With the engine idling and the hood up, use your hand to move the throttle cable. Open it quickly at least half way. Do the RPMs climb quickly and smoothly? Does it hesitate and bog down. Does it backfire?

If it doesn't sound like a healthy "whoosh" of air being sucked in (or as you said in your first post like it is backfiring through intake) my money is still on a timing issue. You haven't said what kind of mileage is on the car yet? Those chains don't stay tight forever!

.
Good advice. Can you measure fuel pressure at the rail? Should be around 40psi at idle with vacuum tube disconnected from fuel pressure regulator. If you don't get healthy pressure, then I would suspect the fuel filter, pressure regulator, or even fuel pump.

I don't think a fuel injector can be ruined by a cylinder misfiring. Although this can mess up plugs. I might also suspect a leaking injector. Before you pull the fuel rail, remove the injector wiring harness from each injector. Use a digital volt meter and measure the resistance across each injector. All should be in the 14-16 ohms range if my memory serves. Again this is a mileage thing, if your car has 80k, then it may be time to replace injectors.

Except the timing chain and fuel injectors. Perhaps ignition coil?

Before you get into reprogramming, get yourself an ALDL cable and Datamaster and do some logging. The computer will tell you what the sensors are seeing. Post your results and there are some sharp guys in the Scan/Tune section that can spot trouble.

We need the mileage to offer more advice.

 

85,000

 

The first thing you need to do is buy a helms book and quit throwing parts at it and that includes any parts others might have told you to throw at it as well.

Second if you have a laptop buy a cable and log some data. You can download tts datamaster, the first 20 data logs are free or freescan (free but not as nice). It will tell you what every sensor on the motor is doing while it is running and the A/F, any knock retard etc... The cable can be had from AKM data cabels for $90 or if you are good with electronics you can build it your self, I have done both.

Scan software:
http://www.ttspowersystems.com/
http://www.andywhittaker.com/ecu/ecu_software.htm

Cable:
http://www.akmcables.com/ic.htm

Build your own cable:
http://www.andywhittaker.com/ecu/aldl_hardware_cont.htm

 

As K99 stated the LT4 (or 96 LT1) and 92-95 LT1 Knock sensors are not the same and will not work and the LT4 KM will do nothing other then spend extra $$$$$ unless you have false knock and even then it is a crap shoot at best. I was sent the wrong KS's when I first built my 383 (LT4 KS's) and it will set a code and cause the motor to run like crap. THe KS is not realy LT4 but rather OBDII -vs- OBDI (The 94/95 are still OBDI for most functions including the knock sensors). THe OBDI sensor is around 4K ohms and the OBDII is 100K ohms

 

Yes I ment the Knock module in the computer ,Or you can take the but end of a screwdriver and tap on the computer and see if the engine stumbles or changes pitch.
I hope you are not referring to all the engine problems you are having,Just boil down to the ASR ABS system?
The engine will back fire and the car will run like crap if it is on and you try to do a burn out or race the car.
You have a button on the dash to turn that junk off.
We are just trying to help, not scold you .I don't know how long you have had your car and if you know alot of things with these cars are normal.
I own a 94 and if the injectors need replacing at 85,000 I think something else is realy the problem.
Here is one for you. I just installed the new Zex spark plugs for nitrous and superchargers ,turbos and boy does the car run like crap.The engine on a 94 always sounds like it is missing .Race week end is over so check your car and get back to us.
If you go to the tts site and download the program make sure you turn off your modem on your computer and all infered devices when logging or you will waste all your free logs( if you own a newer computer,I use an old IBM Notebook) ,even though you can delete the program and download again.

 

Excellent advice. Nothing wrong with putting in new parts but that should be done if you are bored.

 

It was the Head Gasket on the driver side.