Hi everyone, have had this question for years and i figured maybe one  of you autonerdz can answer it for me. Can a variation in cmp/ckp correlation be found with only a voltage RC test in order to find/confirm a slipping crank key/reluctor or missing teeth on timing belt? thanks

On an inline engine it's not possible to spot a jumped timing belt by RC as long as there is still some compression. The jumped timing will affect all cylinders equally....

On a V-engine, depending on its design and the exact type of fault, you may see an entire bank lower than the other bank.

JÜJA wrote on May 13^th, 2024 at 10:21pm:

On an inline engine it's not possible to spot a jumped timing belt by RC as long as there is still some compression. The jumped timing will affect all cylinders equally.... On a V-engine, depending on its design and the exact type of fault, you may see an entire bank lower than the other bank.

I can get behind this as a concept.



frombaja wrote on May 10^th, 2024 at 10:55pm:

Can a variation in cmp/ckp correlation be found with only a voltage RC test in order to find/confirm a slipping crank key/reluctor or missing teeth on timing belt?

I've been noodling on this and while I agree with JÜJA, I also think there are too many variables to consider this a reliable technique. If someone thinks I'm wrong, please teach me.

Don't get me wrong, I've done countless RC tests using current alone, seen little variation between cylinders and moved on. Its fast and can tell us a lot of info but its not perfect. Even when you add a second channel and synchronize to I.D. a specific cylinder, its not perfect. I'm sure others will agree with me that if you have a situation with a broken valve spring you can have perfect RC test and still have a misfire from a base engine issue. BUT, if you have a broken valve spring that caused a valve to get bent and now there is no compression in that cylinder, an RC test is going to show it.

We've discussed the merits of RC tests using strictly voltage instead of amperage in the hands-on classes I've done. IMO, using current is more beneficial than using voltage alone. My reasoning is that amperage can tell us how hard the starter motor had to work to roll the engine over, voltage just tells us that some amount of work was done. Going back to the earliest days of my training we were taught to use a carbon pile load tester for starters, batteries and alternators. I'll list the rules of thumb (That I was taught) for checking the amperage flow through a starter below.

4 Cyl = ~150A
6 Cyl = ~200A
8 Cyl = ~250A

Combining these specs with a concern of a broken belt or major timing chain failure we might be able to identify an issue. For example if I know an 8 cylinder should draw approximately 200 amps average cranking and I only measure 100. That might indicate there is little to no compression on the the whole engine.

Hope this makes sense. Let me know your thoughts.