94 K1500 5.7 timing question.

[Smallz72project]

I recently swapped motors in my 94 with one from a 92 the guy just rebuilt. I installed a brand new distributor, got it running but noticed I had it one tooth off so I fixed that and had a buddy help me set timing. Ever since then, if I drive at low RPM it chucks like it wants to die or has a really lumpy cam. And if I am just slowing down at lower rpms it does it as well. Starts up, and drives at speed just fine on the highway. Another thing I have noticed is my mpg suck!! I expected it to with this truck but Im only getting 9-10...I figure I should be getting at least 13-14. I have suspected timing to be the culprit...does this sound familiar or like I am the right thought process to anyone? Thx in advance and keep on truckin

[Tx Firefighter]

Did you disconnect the wire before setting timing and reconnect it afterward ?

[ChevyTech]

To set the ignition timing you must unplug the set timing connector. The set timing wire, is a single tan wire with a black stripe/tracer on it. From the factory there is what looks like black tape around the harness holding the set timing wire to the back of the harness. Doing this will set a code 42 in the computer. Clear the code after you are done checking or setting the timing.

In 1994 the set timing connector was moved to the interior side of the firewall inside the cab on the C/K trucks. It should be in the area of the computer.

[Smallz72project]

I did disconnect it while setting the timing and reconnected it yes, however I did not clear the code.

[ChevyTech]

Check the coolant sensor the computor monitors, grounds, and the IAC.

Testing information in this thread - see post #7
http://67-72chevytrucks.com/vboard/s...d.php?t=322811

Check for trouble codes.

[Smallz72project]

Pulled the blink codes, came up with 43 which is the knock sensor. I had this same issue with the first engine but it just had a broken wire so I fixed it and it went away. Checked the knock sensor visually and the wire is still in good shape. Cleared the codes, started the truck, light was out for 30 seconds and then came back on. Not really sure what that chart is saying should be normal or when to test it. It is possible that may be bad though...had an overheating issue due to a bad waterpump. Fixed the pump but temp always seems to read low on the temp gauge. I will replace it and look at the knock sensor closer and see where it goes.

[ChevyTech]

Once the code 43 sets during a drive cycle the system will run in a backup mode, and there will no electronic spark timing control which will affect the way it runs. This will cause poor performance and bad fuel mileage.

Did you remove the knock sensor from the 1994 engine and install it in the 1992 engine when you did the swap?

Test the resistance of the knock sensor and let me know what you find.

[Smallz72project]

No, I did not swap the sensor over, I just ran the one that was on the 92. How do I test the resistance on it since its a one wire sensor? Just test through it (one lead on the wire, other lead on the portion that threads through the block)?

[ChevyTech]

Originally Posted by Smallz72project

No, I did not swap the sensor over, I just ran the one that was on the 92.

The 1992 had a 160 Baud rate C3 ECM with a separate Electronic Spark Control module.
Your 1994 has a 8192 BAUD PCM with the ESC built into the computer.
I don't think the sensors are compatible with each other

Originally Posted by Smallz72project

How do I test the resistance on it since its a one wire sensor? Just test through it (one lead on the wire, other lead on the portion that threads through the block)?

Yes

If you still have the knock sensor from the old engine, compare the resistance of it to the one in the new engine.

If you don't have the other sensor, let me know the resistance of the one you have in the truck now.

[Smallz72project]

Okay I pulled the one out of the 94 engine. Resistance when tested on the 20k ohm setting was 3.92 ohms. Was not prepared or expecting coolant to gush out lol...needless to say I have a bit of a mess to clean in the shop.

[ChevyTech]

Yes, the knock sensor screws into the block drain hole.

I am guessing that is 3.92K ohms, or right at the 3900 ohms expected. Ohms meters can be hard to read.

I bet the 1992 sensor in the truck measure 100K ohms.

If so, swap them out.

[Smallz72project]

I have to say a big thank you to you ChevyTech. Bit the bullet and stayed a few minutes after work today so I could get my 94 up on the hoist and swap the knock sensor for ****s and giggles, I got soaked in coolant!! But, the check engine light shut off right away and the truck is running SOOOO much better. Still has a little bit of a hiccup at lower RPMs while under load, but no where close to what it was like before. Once I got home I did test the ohms of the 92' knock sensor and you hit it on the head at 100 ohms. Computer is obviously going to take a few driving cycles to relearn itself and have it completely out of its system from driving so long in backup mode. I still think I am going to recheck and adjust the timing again after this but I think the main culprit has been fixed, all thanks to you! Gotta love this site!! If there is ever anything I can ever help you figure out I am glad to oblige. Thx again and keep on truckin!

[ChevyTech]

Glad to hear things worked out OK, and thanks for sharing what you have found.

I have tried to help so many people on this same issue and very few of them will follow what I suggest be done.

The parts look the same but are not compatible.

Good job, and you're welcome!

If it does not clear up, post back on this same thread and I should find it.

[Smallz72project]

So today...it seemed to run like crap just like before. I checked the timing tonight and it was good, with the set timing connector disconnected, and motor warm it was 2-3 degrees before TDC. I did find a vacuum line had broke and melted that goes over to the canister on the front left. I replaced it with the one off a 95 chevy. Im getting very stressed and discouraged with this heap. Would the water temp sensor on top of the manifold be the same one as a 1995? As in me pulling the one off the 95 engine and using it? Im just to the throwing parts at it stage...luckily it just happens that I have a parts truck with the same set up.

[Cabnchassis]

Temp sensors are pretty cheap and since the ECU relies on the information they put out I'd replace the one for the ECU with a brand new one. Should be in $15 range. Also as chevy tech suggested test the IAC valve, it can cause a lot of low idle problems. You might also want to check around the TB base and the Intake manifold gaskets for vacuum leaks.

[ChevyTech]

Sorry I am slow at getting back to the thread. I have been busy.

A coolant sensor (for the computer - by the thermostat) from a 1995 would work in your truck. The same coolant sensor is used on many years and engines.


Is it setting code 43 again?

Check for codes even if the check engine light is not on.

The Knock sensor contains a piezoelectric crystal which causes a voltage to be produced. Loose components on or in the engine can cause the knock sensor to create a voltage and the system may interpret this as pinging. The system retards the timing because it determines it is pinging. Loose torque converter bolts can cause a false knock signal to be detected.