Part-throttle knock, I'm stumped


New Member
Sep 18, 2003
I've been getting this part-throttle knock since I bought this car, and I've tried everything I can think of, to no avail. Usually, it's less than 5 degrees, sometimes goes higher though. It's like usually when I'm changing lanes, going up a hill or something, but far from WOT. Sometimes, I can hear the pings too, so it's not false knock.

Used to be, KR would go away when I really got in it all the way to WOT. Lately (past month), the KRs seem to hang around longer than before, sometimes even when I'm at WOT. It's been colder here lately, and I recently have gone to biggest propane nozzle to overcome colder propane tank - those are the only recent changes.

Engine has about 60K on it - FPR is set at 48psi (line off), boost recently lowered from 22 to 17 since KRs have worsened. Translator still set to 8% rich at WOT. I have experimented with Propain turn-on point, DC, and nozzle, but nothing has improved (with propane, I can generally run higher boost of course, but part-throttle KRs remain). 02s show about 790-800 when KR is highest - I don't know whether to trust that with the propane, have bought an EGT gauge, but not installed yet.

Suggestions I had heard so far, have been addressed:
EGR is working. RJC Power Plate installed. Catch can in PCV circuit to prevent most of oil to intake. GM Top Engine Cleaner used recently, no noticeable difference.
Other suggestions I haven't verified are: Cracked Header? Cam lobe wearing out?

I had a 28* Thrasher in there, but have recently switched back to an 18* chip, then to stock chip, and although total KR is less now, it's definitely still there.

Very frustrating - the car still pulls pretty good, but I'm afraid to get into it too much, lest I break something and be really sorry.

Anyone else have these symptoms, or have any suggestions? Thanks, and Happy Thanksgiving.

I would start by turning the propane off and try tuning it without it. Use the 18degree chip if its from a quality vendor. Turn the boost down to 10 pounds or so and see if that gets rid of the knock. You want to start low and work your way up not the other way around. 17#can be a little high on 93oct for some cars. Run the car in 3rd gear from 55-60mph on up to 100+mph. This is a good area to tune in because if you have no knock in this range you are probably all right. If no KR then raise the boost a pound at a time till your KR comes back. Once your running as much boost as you can without KR then try to lean it out a little bit. 1 or 2 pounds at a time till you see some KR then back up. At this point you can turn the propane on and see if it lets you run some more boost without any KR. Your fuel pressure also seems a little high. Most chips are set to perform at between 43 and 45 psi with the line off. If you get it down to 10#'s of boost and your still seeing KR then you have a problem somewhere you will have to track down. Also if your not able to tune to a decent level of boost you probably have a problem somewhere but start with the boost low and work your way up to play it safe. I would think you could tune the car to run at least 18pounds of boost on the street with 93oct and the propane with stock heads. Also take the 8% adjustment out of the translator, set it to stock if thats possible. Try this and let us know how high your able to tune it and then we should be able to tell you if you have a problem. Also confirm that your boost gauge is accurate if you have an air compressor. Good luck and hope this helps.

I had the exact same trouble with my 87 GN, part throttle knock with the cruise on pulling a moderate hill, or light acceleration at highway speeds. It was there when I bought it with 57,000 miles on it. I was lost but ended up finding the problem completely by accident. I was replacing the cam and timing chain and noticed that the EGR ports in in the heads were completely pluged up with carbon. I just cleaned them up with a screw driver, make sure the valves were closed,(I had the cam out at the time), and cleaned up the mess with a long thin funnel and a shop vac. When I got it all together, the part throttle knock was gone for good.......none at all!!
If you have a chip which is set up for EGR, but the EGR is not working, that can cause part-throttle knock. I don't know about the other chip, but the Thrasher is set up for EGR, and the stock chip most certainly is. Did you drill out the EGR hole when you installed your Power Plate?
Unplug your TCC at the transmission and try it. Your chip may be using Stock-like command of the TCC which makes it wait until quite a high TPS value before unlocking. On my car that made a huge difference. In 4th the boost would get up to 10 psi and start knocking until the TCC opened up and the engine could rev up a bit. The Extender chips for instance let go of the TCC very quickly and it made a big difference for the part throttle knock, as well as driveability.