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79 toyota pickup engine problems

OK, well i have an old toyota pick up, One of the spark plugs ( the third cylinder) has some oily residue on it, indicating a problem with the piston. How bad could this be? Is it worth it to try and have fixed? I also am rebuilding my carb right now, so i am wondering if i is even worth continuing if the engine is messed up. any advice, experiences would be great.

Test the compression on all cylinders. If none are under 110 psi and oil consumption is less than a quart/1000 miles I’d recommend repairing the carburetor and keeping the oil topped off. But keep in mind that I am one of those who tries to get every good mile out of vehicles that is possible.

If the compression is low in that one cylinder it may be due to a bad head gasket or piston rings stuck due to carbon. Yamaha I think makes a cleaner. In my boat the repair guy did a head gasket replacement, due to low compression in one cylinder, the old head gaskets looked good, he then used some kind of cleaner that blasts out carbon and frees up the rings. It was a stab at the cat to avoid an expensive engine rebuild. It worked. Poor start when cold was my initial problem. It would take like 10 tries of start until it would stay running. It starts very well now and I think I may have picked up an extra 5 mph top end. If only the speedometer worked.

I had a 79 with the 20R engine. RIP - frame broke in 2007.

I rebuilt a couple carbs for it. There is a solenoid on the back right side of the carb. It was not working (no click when 12 V were applied/released) and that caused major flooding, IIRC. Replaced that with one from a junkyard carb.

The AAP (auxilary accelerator pump) diaphragm is prone to leaking. It will cause flooding and oil dilution. I replaced two or more over the years. Eventually I disabled the AAP altogether by blocking off the vacuum line. Truck still started and ran fine.

I agree with the earlier advice: do a compression test. Tell us your results.

You can pour half a can of penetrating oil in the cylinder and let it soak overnight and it could free some gunk that is sticking the oil ring. Blow it out of the cylinder the next day. Then forget about it and keep driving. Don’t forget to change the oil and filter after idling the engine for 15 minutes.

I also had a '79 Toyota pickup, and like shanonia my frame rotted in half…after 10-1/2 years. Before you continue doing your repair,s let me suggest that you examine the frame for rot. Typically these will rot out first at the points just aft of the front bed mounts. If the frame is rotted there, the vahicle is unsafe and beyond saving.

The oil problem is, as you suspect, likely due to tired old oil rings. In addition to wearing (along aith the cylinder walls), they eventually lose the spring tension that keeps them pushing with the proper force upon the cylinder walls. And they can also become gummed up over time.

By all means do a compression test. But understand that even if the compression rings test okay that does not mean that the oil rings below them are still good. The oil rings could be tired and leaving too much oil on the walls, allowing it to get drawn up into the cylinders during the high-vacuum intake stroke and burned. Compression rings are designed to press against the walls when top-loaded, as they are during a compression test, but during the intake stroke the pressure abive them is substatially lower than the pressure below them, and excess oil left by the oil rings on the cylinder walls can get drawn past them. The problem can actually be exascerbated by blowby pressurizing the crankcase.

In short, your oil rings are probably shot. But before you spend any more time or money, I’d suggest getting under the vahicle (safely, please) and chceking it carefully for rot and for related safety issues.

And honestly, unless this truck is in exceptional condition, I’d be inclined to just kepe it running and change the plug(s) more often than normal no matter what teh compression looks like. The value of the truck doesn’t justify spending a lot of money on it. It can keep running for years in the condition it’s in.

thanks to all who replied. i did a compression test and the cylinders seem fine. I cant figure out what else could be wrong with it. The carb is not in the best shape, i did not yet rebuild it. It has sparks to all the plugs and gas in the engine but it will not start. a friend thinks i might have jumped a tooth on the timing chain. It is a double chain. HAs anybody ever had this happen. how do you tell. i could not find the bright link on the chain. it seems pretty unlikely that it would jump a tooth, since it is a double chain. I cant figure out what else it could be.