My son has been driving a 1992 pickup this summer on a cross country work assignment. Before he left, the heads were rebuilt because of a burnt valve. At that time we were advised to replace the knock sensor, but the mechanic who did the work did not do this and my son had to leave the minute the truck came out of the shop.

Now he is on his way back, and he's driving about 2,000 miles to get home. the check engine light has been coming on a fairly regular basis ever since he's been gone. When the check engine light comes on, the truck will not go over 75 MPH. The light will eventually go out and then the truck will perform normally again.

Obviously he would be well advised to have the code pulled, but since he's on the road right now, I wondered if anyone here had an idea of what this could be.

Only takes a minute or so to pull the stored code(s):
http://www.4crawler.com/4x4/CheapTricks/index.shtml#TimingCheckConnector

And you probably already have a code reader:

uhhhhh... how often does he need to go over 75mph?
if i told my dad my truck would act funny when im trying to go over 75mph he'd kick my @$$... (speed limit 62mph here)
guessing its a bit different down there?

I'm with Greg - going over 75mph is really not required...

The engine code is related to the Knock Sensor. It is a bit amazing how lazy/uninformed some mechanics still are, especially with the internet. It is well known throughout the Toyota community and most mechanics that work on Toyota's that you must replace the knock sensor/KS wire and how you must use new head bolts when working on the 3.0. The KS wire runs right along the head and gets brittle from the heat, and is why it needs to be replaced, Toyota recommends replacing the KS as well due to its location.

The reason it wont run over 75 is the ecu isn't getting any feedback from the KS and retards the timing to prevent engine damage at higher RPM's, it should normally come on at about 2500-2800rpms.

I would suggest he pick up a code reader for about $30 and check the codes himself. As long as the code comes up for KS, he should be fine, just tell him not to run the engine at high rpm's until he gets the KS replaced.

When he gets back, take the truck back to the mechanic and demand he put a new KS and KS wire in, and if he didn't use new headbolts demand he install new ones. The 3.0 head bolts are torque-to-yield bolts and are a one time use only bolt. The reason for this is that the bolt is designed to stretch during the torque process(diff process than normal head bolts), the stretching process makes it impossible to get the same torque value once they are loosened, and reusing TTY head bolts is a guarantee of head gasket failure in the short term future. You should expect to pay for the KS/KS wire and new head bolts(if nec.), but not the labor to install, labor cost is the mechanics loss for not properly rebuilding the engine to start with.

Thanks to everyone for the replies. The truck was running so poorly that I had to fly to Denver to help get him home. As it turns out, when the CEL came on, the truck would usually not exceed 55 MPH. Felt exactly like it was running in failsafe mode with the timing retarded. No power whatsoever and had to literally floor it to maintain any speed.

The ECU stored three codes:

14--Ignition circuit
41--Throttle position sensor
52--Knock sensor

We replaced the TPS and the coil, but truck still ran terribly. You mentioned that the knock sensor wire runs along the head, but where exactly is it? Left or right bank? Front or back? I'm going to try to look at this tonight. It's the only thing we didn't replace. Any idea of what one of these costs and is Toyota the only source for the part?

I made sure the mechanic used new headbolts, by the way. He already knew they were TTY in this application.

There is no need to replace a TPS, it just has to be adjusted. i believe on Roger Brown's site www.4crawler.com , he explains how to adjust it.
I also believe the knock sensor is under the intake... dunno for sure, but you have to order the sensor and wire, and yes, directly from toyota. youre supposed to replace it every time you touch it (yes sadly). I've head of the 4cyl guys using a chevy 350 knock sensor because it's not as fragile. dunno if it will fit the v6...

Look for the plug to the 'intermediate KS wire' just under the TPS, its round and is sandwiched between the intake manifold and the cylinder head.