Engine would knock after it's warmed up and under load (going up hills or elevated roads)

SHANGHAINIGHT

MEMBER
1991 TOYOTA COROLLA
100,000 MILES

My dad's car, it would run fine when the engine is cold, but after maybe 5 miles and under load (going up to the elevated road and if I accelerate) I would sense light knocking from the engine, this knocking feeling would be more after 20 minutes of driving (approximately) when on the flat road and when I want to accelerate, I had to ease back on the gas paddle to avoid it.

It's been to a mechanic for this problem, they were not able to really find the cause and fix it (old car with older code system, they were busy fixing Mercedes and Bimmers. ), They eventually retard the ignition a bit, but the problem's still exists.

Backgrounds:
I have not noticed the temp. Gone up when the knocks happen.
It runs fine when the engine is cold.
Strange thing is the cooling fan would go on when I switch the ignition to ON (before I crank the engine) in the morning, there must be something wrong, but don't know what.
I have no knowledge of it's service history.

4 Replies

KHLOW2008

MECHANIC
41,815 POSTS

For the fans running with ignition switch turned on, check the fan switch. If it is unplugged or has a broken the fans would be running with ignition switch turned on.

As to the knocking, get the ignition timing checked. If initial ignition timing is too advanced, retarding it slightly would not eliminate the problem. Is the MIL indicating and did you check for trouble codes?

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Tuesday, July 3rd, 2012 AT 3:00 PM

SHANGHAINIGHT

MEMBER
3 POSTS

If the fan switch is unplugged then where's the power to make the fan turning? Sorry, I don't understand this. Unless the fan is powered directly off the battery and the switch there is to disconnect it when the fan is not needed?

Is MIL the check engine light? No, it was not on.
I have no access to a repair manual, don't know where the ECU is, if someone would tell me where to look for, I can try to retrieve the trouble codes.

I do know that the mechanic cleaned the carbon deposit on that visit for this problem, (also did the maintenance, changed oil, air filter and such)

Thanks HKLOW2008!

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Wednesday, July 4th, 2012 AT 4:50 AM

KHLOW2008

MECHANIC
41,815 POSTS

Here are the schematics. Toyota uses a different system than others and the fan switch id for toggling the radiator fan relay. I understand why you are puzzeled.

The fan switch circuit is permanently grounded until the operating temperature for the fan to turn on is breached. When the fan switch circuit is broken, the radiator relay ( it is an A relay whereby it is permanently ON when the coil is not energised) would kick in and turn the fan on.

The other things you need to check would be the fan relay. If the wrong one is used, you would have a reversed problem, fans working when temperature is low and cutting off when the fans are supposed to turn on.

A fault in the A/C high pressure switch or a break in the wiring between relay, A/C high pressure switch and fan switch would cause the problem as well.

ECU is behind the radio but you don't need to get to it to retrieve trouble codes.

https://www.2carpros.com/articles/toyota-trouble-code-definitions-and-code-gathering-method-1990-1995

The above link would show you how to check for trouble codes.

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