Hello,

Hoping someone can clear this up for me. My 94 LT1, with OBD II, is now a 383 stroker. I notice when steping on the gas, the timing is retarded due to knock. ( 3 to 5 degrees with just slight throttle) I think the noise is from the cam and the rebuild, so can I drop in a LT4 knock sensor, will that help?

Otherwise, do I need to get the computer reprogramed (again) to eliminate this knock retardation?

Thanks

 

Need the details of your setup. Did you do an OBD-I -> OBD-II conversion? Why? The 94 PCM is OBD-I. If you did convert from OBD-I to OBD-II, and are running an OBD-II PCM, do you have the correct knock sensor in there? The 93-95 OBD-I knock sensor is 4.5Kohms, the 96/97 OBD-II knock sensor is 100Kohms. Use the wrong sensor with the PCM, and you get a trouble code, and the PCM starts retarding the timing a variable amount almost all the time. Do you have any codes? Is the SES light on?

If it really is knocking and causing retard, you need to find out why its knocking. Are you using the correct fuel? What is your static compression ratio? What is your dynamic compression ratio? Does the engine run warm at all? Do you have the rear coolant cross-over tube in place? Do you have full roller rockers? Hopefully, not a solid roller cam.

There is no such thing as an LT4 knock sensor - the LT4 uses the same OBD-II sensor as mentioned above. What you would need to eliminate false knock due to noisy valve train, you could install the LT4 knock module to try and eliminate it.

 

Injuneer,

Thank you for the quick reply.

You are right, my mistake, it is OBD-I. It has been awhile since I've tinkered with this engine.

Right now, the knock sensor is the original, stock unit. The computer has been reprogrammed for the 383, but I would imagine only for the fuel mapping. There are no codes, SES light is not on.

I am running 91 oct gas, and if I understood my engine builder correctly, the compression ratio has been maintained to the stock ratio. I had my scanner before the rebuild, and the motor definitely did not have knock retard before the build.

The engine does run warm, as it is in a Toyota truck body, but not exceptionally warm. (water temp at the heads at around 210 to 230 when fully warmed up) but even when the engine is warm or cold, it has this knock retard.

The cam is a competition cams, I don't remember the number, but it is a mild cam for torque applications, to match the 383. But is was mild enough to not create a loopy idle or give me trouble passing CA smog. Otherwise, everything else is stock.

With the 94 LT1 and stock computer, can I swap out the knock module, or is that built in to something?

Thanks for you help!

 

Oh yeah,

I forgot. Yes, the rear cross-over tube is in place, as is the steam vent to the radiator.

Otherwise, this is a totally stock LT1 motor shoe-horned into a Toyota truck body.

Thanks!

 

Does it show any knock retard at idle?

With the tight quarters, is there anything that the engine is hitting against on the chassis or suspension?

Is there possibly a high voltage wire (e.g. plug wire) too close to the wire for the knock sensor?

Are your motor mounts tight, and no possibility of any rattling? Obvioulsy the KS is right next to the stock motor mount pad and any problems with the mounts will be picked up by the KS.

 

Ok, lets see...

No, no knock retard at idle. At idle it is exactly 0 deg, and the regular timing is about 27 deg.

I understand your questions with the mounts, as any exterior knocking will set it off, but the motor is mounted securely, in exactly the same way that it was prior to the rebuild. Because there was no knock retard before the rebuild, then either I installed something differently, or something is different internally.

With the truck smoothly idling, if I step slightly on the gas, and run the tach up to about 1500 to 1800rpm, the motor isn't rocking or moving significantly at all, it is still smoothly humming along, but the knock retard is already showing 3 to 5 deg.

The valve-train was redone, as were the heads, by the machine shop that built the motor. Couldn't the slightly higher cam be causing the internal noise? Is there something about the longer stroke of the new crankshaft that could be doing it as well?

Thank you, I do appreciate your attention on this. I don't know that much about specific LT1 problems, so your help is greatly appreciated.

E

 

Your crank or rod bolts could be hitting the block or the pan if they didn't clearance it properly, but that noise wouldn't last too long, and would exist even at idle.

You also might want to check to see if you indexed the Opti correctly. Get it off a tooth, and you could have a lot more advance than the PCM is indicating, since the PCM is showing you the numbers its pulling out of the tables, not the actual advance.