smuryof

Bad valve diagnosis

10 posts in this topic

Hey all,

Please check my troubleshooting; I believe I have a bad valve. Have I missed any steps?

Symptoms:

  • Heavy misfire under load <3Krpm. All gears, esp. from a start
  • Rough idle

Analysis:

  • Check TPS for correct resistance from Closed to WOT - passed
  • Check MAF for correct resistance - passed
  • Replaced aged O2 sensors (they were quite old) - no change in performance
  • Checked compression; At normal operating temperature, 145psi 110psi 150psi 141psi (from left-to-right) By my calculation, that's a 27% deficit on cylinder number 2.
  • Warmed up engine, removed oil filler cap, and then dipstick, while running - no smoke or any sort of pulsing from crankcase can be observed
  • Removed valve cover, examined cam lobes - at first thought I had a worn lobe, but they all check out OK.

Have I missed any troubleshooting steps, before I start removing the head for a valve job?

Thanks!

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I invested in a leak-down tester to pinpoint the low compression, and discovered worn piston rings on what appears to be multiple cylinders. Pretty cool test, and very easy to do on these engines. Unfortunately for me, this means I have to pretty much rebuild this engine. Maybe later when I have the time.

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Have you ever had one of those Homer Simpson D'OH! moments? I did yesterday.

Here's a pro-tip: If you change your timing belt, set your timing afterward!!! I didn't have my timing light with me on this road trip, and I guess I thought since everything went back the same way, and I didn't touch the distrib. or remove cams, that the timing wouldn't change. Not the case. My timing was 10 degrees late (retarded) and that was greatly exacerbating a weak spark.

And yes, I mention weak spark because the other D'OH! was, even though I was seeing a white spark at the plug, I never checked the coil windings in the distributor. I have proper resistance on the primary, but the second coil is broken, registering infinite resistance in the test. So, on top of bad timing, I have a broken coil. I will be putting a distributor in tonight - a $300 part and 5 minutes of labor.

Anyhow, the car is running much better & I won't be rebuilding the engine anytime soon.

Moral of the story - with a heavy miss under load, always check your timing! 

At least now I have a *very* thorough understanding of the health of my EGR valve, fuel pressure & vacuum-driven fuel regulator valve, valve seals & healthy cam lobes, cylinder walls & normal piston ring wear, lack of air leaks, charcoal canister function, O2, TPS & MAF sensors, and all ignition components. And I have a s***-load of diagnostic tools :)

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For those of you following my saga (yes I know there are MILLIONS of subscribers) --- I replaced the distrib. today. I gotta say, any hesitation is totally gone. The car runs freaking GREAT. Well, great for 230,000 miles - still has some funny idle up/down action at stops, but nothing I am worried about. Puh-LENTY of pulling power up hills in low gear, or anything else.

 

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IAC can also cause these issues.  I'm dealing with stumbling under load myself but it idles fine just a little high.  Idk what else to check after I clean the IAC and it doesn't work.  Good luck 

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7 hours ago, qualzoom_97 said:

IAC can also cause these issues.  I'm dealing with stumbling under load myself but it idles fine just a little high.  Idk what else to check after I clean the IAC and it doesn't work.  Good luck 

Did you check your timing and ignition coil?

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No I don't have a timing light but it could be replaced because of age alone.  It idles smooth but I have to feather the gas and it does fine.  I checked the MAF and the volts stay at 5. It doesn't drop when opening it.  Hope this is it and hope I done the check correctly. Thanks a lot.  I will get the timing and check the coil.  

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I don't know if we're allowed to advocate products on here - but Harbor Freight sells a cheap timing light that works just fine. Your symptoms sound exactly like mine when I was ten degrees off on timing. Bring a paperclip and do it in the parking lot on the spot :)

Resistance check for the primary and secondary coils is super easy and only takes a couple minutes. Let me know if you need a reference to the workshop manual for it, you can find links to it in other threads I've responded to also.

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13 hours ago, smuryof said:

I don't know if we're allowed to advocate products on here - but Harbor Freight sells a cheap timing light that works just fine. Your symptoms sound exactly like mine when I was ten degrees off on timing. Bring a paperclip and do it in the parking lot on the spot :)

Resistance check for the primary and secondary coils is super easy and only takes a couple minutes. Let me know if you need a reference to the workshop manual for it, you can find links to it in other threads I've responded to also.

You are ;)

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