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RB26 - Compression and Leak Down Test Questions

Practical Engine Building

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Hi all,

My RB26 has a misfire on cylinder 6. It sounds and feels like it's only firing with half the power it should.

I've already changed the coil pack harness, coil packs and spark plugs. Also taken out, cleaned and tested the injectors. I've checked the voltage at the injector and that is also good and then checked for a good spark.

The one thing I don't have at the moment but have on order is a Picoscope in the hope that will enable me to check the pulses to the injector.

I ran a compression test which came back with 160PSI across the board and then a leak-down test on all cylinders which comes back as 5-8% across the board. The only difference is that cylinder 6 then slowly loses pressure. It's around 5 PSI per minute. The leak sounds like it comes from the intake valves.

Any pointers on what I can check next or what the issue might likely be?


Take this as a sounding board/discussion points that may trigger a line of thought for you - and hopefully others will chip in with suggestions/corrections.

If your ECU allows it, or doing in manually, you can try deactivating injectors or ignition and logging the rpm drops at idle as each is dropped in turn.

You say you cleaned and/or changed the ignition components and injectors - did you swap around the injectors and coils, too - it's unlikely, but that may show something?

How did the spark plugs look - any differences in deposits, if any, colourations, etc?

You can also buy cheap boroscopes, some even cheaper basic cameras work with smart 'phones, and have a good look in the cylinder - some come with mirrors that can be used to get some idea of valve condition, too. If you remove the inlet manifold, you can also get a good look at the back of the valves, and compare them to the other cylinders - variations in colour, carbon /other deposit build-ups, etc, may help direct the diagnosis? If you rotate the engine to the points where the intake, then the exhaust, are fully open, a boroscope should also allow you to see what the seats are like, between inspecting through the 'plug hole, and from the port.

NOTE, make sure any mirror, or other end attachment is secure - you don't want anything dropping off into the cylinder!

It's rather unlikely, but check the valve clearances, as if they closed up the valves may not be seating perfectly. It's also unlikely, but there may be a weak, or broken, intake valve spring - but the cylinder pressure during the leak-down should be holding it firmly closed.

Worn intake guides could mean the valves aren't perfectly seating.

Most of that may not be relevant, though, because the compression test and cylinder leakage is pretty darn good, and I would expect it to need to be a lot worse for a drastic loss of power on one cylinder.

All in all, if you're SURE that cylinder is weak, it may need to be pulled down - but before that, about the other big variable may be if one, or more, of the #6 cam' lobes are badly worn?

[edit] I expect you've already ruled out a weeping head gasket?

Gord presented great info.

Stewart I'm hoping for some clarification on something you said. If all cylinders have 5-8% leakdown, they're all going to drop pressure once the pressure source is removed. It wouldn't make sense that only one cylinder would have leakdown, if you found all of them have 5-8% leakdown. Can you clarify what you meant?

Please also let us know how you determined cylinder 6 is misfiring. That info may help us give better suggestions.

Your test results are typical of a high mileage engine that's worn, but not broken, so while the engine could be the issue still in some fashion, I'd investigate the external items like Gord mentioned first i.e. spark plugs, swapping coil and injector locations to see if there's a change.

Thanks Gord. Yes, I tried swapping the injectors and coil packs around. I also checked all of the cylinders with a boroscope and I couldn't see any issues.

After more diagnostics today I have some suspicions about the Power FC. I'm waiting for a Picoscope and the suspected power transistors inside the ECU that drive the injectors, particularly cylinder 6.

I don't recall offhand if possible on that particular ECU, but in a pinch I've altered firing order and swapped ECU outputs or injector/coil harnesses so the engine does fire in the proper order, but you're able to run cylinder 6 off a different set of outputs and see if the misfire follows.

Good idea, Mike, if there's sufficient harness length, perhaps advance or retard the offset by 120 degrees, and move the connections as required - possibly using jump/patch leads.

Yup if items in the engine bay won't reach, swapping pins at the ECU generally works since those outputs are usually near each other.

Hey Stewart,

I'm also curious about how you determined that your issue was on cyl #6. Is it a hard fault (the engine is running on 5 cylinders at all time) or it runs on 6 cylinders at idle and then missfire under load?

Thanks for all the suggestions, very helpful.

Yeah, a hard fault with cylinder 6 only half firing/very weak or not at all.

I found quite a leak from the intake manifold gasket around no.6 so am in the process of replacing all of those. Pig of a job though!

Good work, I'm glad you've found something. Hopefully that's the whole cause.

Thanks all. Job is complete and it has sorted the misfire so that's great. What's not so great is that the throttle bodies are also in a bad state that I didn't notice while doing the job so it all has to come off again! The joys of a 32 year old car!

Great job sticking with it!

Thanks, important to keep these historic cars on the road.

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