1995 Toyota 4Runner erratic ignition timing

Hi davidfarrand,

Thank you for the donation.

Erratic ignition timing can be cause by many factors and you have covered some of them.

Did you check the valve clearances?
Is the idling speed stable?

I did not check the valve clearances. The block and heads were done by a reliable machine shop we have been using for years. The idle speed is stable.

If the engine had just been started after rebuilding, let it run for some time to settle down before rechecking the ignition timing.

If timing is still unstable, check the valve clearances. Sometimes mistakes do occur while working on it.

The engine has 2700 miles on it, I have been wrestling with the timing since November. Should I check valve clearance on both sides or just the distributor side? If the clearance is ok should I look for something else while I'm in there?

I would suggest checking all the valve clearances.

While you are there, it would be easier to have a look at the timing belt synchronisation, so give it a go over to ensure it is correct.

Check the spark plugs as well, compare them to see if they are evenly worn.

Valve clearances are fine, timing belt is right, and no difference in the plugs. I did check compression while the plugs were out. Warm, the cylinders were120+/- dry and 170+/- wet. I don't know what is going on here. I was real careful with the rings during assembly.

What were the ring gaps measurements?
Were pistons new?
Was engine block rebored?

I do not know the ring gap measurements, they were sized at the machine shop. The pistons were not new and the block was bored .020 over.

One of these could be out of specs causing the low dry compression.

When the dry compression test was made, was the engine not running for an extended period of time?

Did you retest the compression after running the engine again?

With 2700 miles on the engine, there is a possibility the block has not run in yet and that couls be the cause of the low compression as well.

What grade of engine oil is used?

The dry compression test was run as soon as the engine had cooled enough to pull the plugs. The oil we are using is Chevron Supreme 10W-30.

Do you have any more thoughts on the erratic timing?

Thanks, David

The normal compression is 171 psi.
Minimum compression is 142 psi.

The engine is not running as good as it should be and this would cause power losses. When this occurs, the ignition timing would fluctuate.

Something is causing the low compression which in turn is affecting the erratic ignition timing.