Rebuilt 3-cylinder Miata motor (long)

[CraiginTX]

So my oil pump failure last fall killed a couple of rod bearings. I pulled the motor and dropped it off with John Day for a rebuild. Bored .020 over, new pistons, rings, bearings, exhintake cam swap, 5-angle valve job, block and head shaved to ensure flat surfaces, etc...

While he had the motor, I replaced all the water hoses, the front trans seal, the clutch, and a few other sundry items. I installed a new header - and have a 3 month-old Magnaflow cat.

I re-installed the motor and fired it up. It seemed to run well, but had way too much HLA noise. I tried idling it, driving it gently around the neighborhood, nothing helped. I talked with John and we decided that re-using the old lifters may not have been the best choice - so I bought a new set of lifters and he installed them.

After he installed the lifters, he started the car idled it a bit, then took it out for a short drive. When he first started it, he says it was running smoothly - and he did let it idle until it was warm and the idle dropped to it's usual 800-900 rpm. After a short drive, it started idling very roughly. As it was late on a Saturday and he had birthday dinner plans with his wife, I agreed to get the car home and we'd tackle it again on Monday.

I gently drove the car home (3 miles) and when I got there it was running perfectly. It sat for a couple hours, then I fired it up again, set the ignition timing, and took it out for a spin. It seemed to be running great, but after driving it for 20 minutes or so, it was running very roughly again, and has ever since. (A week and a half) So here's the diagnostic work done so far:

Air:
* New filter, OEM intake system
* Problem seems to be confined to cylinders 3 & 4, with 4 being much worse
* Valvetrain is now quiet, and when it was running smoothly - it seemed to be running quite well
* Gary and I checked leakdown - 8% to 10% across all 4 cylinders
* Gary and I checked compression - #1/175, #2/165, #3/165, #4/150
* At idle, vacuum gauge reads ~15 in. Hg - vibrating +/- ~1"
* Checked for vacuum leaks and found none
* Intake manifold bolts are tight and I used a new gasket

Fuel:
* Certainly have fuel pressure, though I haven't measured it
* No visible (or sniffable) leaks at rail
*Using my stethoscope, I think I can hear all four injectors clicking, even when it's idling poorly
*When the car is barely idling, I can unplug the #4 injector and there's no change. If I unplug #3 injector, there's little to no change. Unplug #1 or #2, the engine will die
*I've pulled the injectors, cleaned them, replaced the o-rings and the "insulators" that seal them to the head, and intalled them back in different holes - the problem stayed on cylinder 4
*No obvious visible damage to injector wiring harness - wiggling, pushing, pulling on the connectors and harness itself while the car is running doesn't seem to make any difference
*I checked the wiring to the injectors from the ECU - it's continuous
*I also checked to ensure the switched ground lead wasn't continuously grounded

Spark:
*Used a "spark checker" and verified spark at all four cylinders
*Plugs were new with the rebuild, have only 50 miles or so on them. John swapped 2 of them trying to diagnose the problem on when it first occurred, but the problem stayed with #4
*Pulled the plugs and 1, 2, & 3 look like new plugs 4 was BLACK, like it was fouled
*Relatively new Magnecore wires, correctly installed
*Swapped in a set of NGK blues (with under 1000 miles) from a friend's car, no change
*Gapped and installed another set of new plugs - no change
*Swapped #1 & #4 wires at the coil - no change

Timing
*John installed the (new) timing belt after the lifter replacement - feels confident of valve timing.
*Yes, it has the exhintake cam swap, but it HAS run smoothly since the swap.
*After I brought the car home, during the time it was idling smoothly, I adjusted the timing with my timing light to ~12 deg
*I have tried adjusting timing over a pretty wide range while it's idling poorly and while I can tell I'm making a change, nothing seems to help the 3-cylinder thing
*I've checked the wiring to the CAS: +12, +5, +5, GND
*I've also verified the air gap on the crank angle sensor, FWIW

OBD2
* I have had a CEL a couple of times during this fiasco -- code reader said P0300 - random multiple misfire
* CEL will clear itself if left alone
* O2 sensor readings are jumping around a lot, like incomplete (or not at all) combustion in at least one cylinder (tell me something I don't know)
* new Bosch O2 sensor in the header - older one (don't know if it's orignal or not) post cat

Other:
Gary and I put a thermocouple on the primaries of the header and found that #1 & #2 are hot, #3 cooler, #4 cold (relatively)


Anyone got any bright ideas?

[Radio-Active]

Sound like you may have got the ring gaps all aligned.

This would cause oil fouling and a lower compression, resulting in a cooler cylinder.

[MadMerv]

What, exactly, is the cam swap? I doubt it has anything to do with the problem, but I would like to rulle this out. Are the plugs coming out of #4 dry and powdery(carbon) or oily? If you put a new plug in, does it run ok for a while? THe compression check bothers me for a rebuild, but the leak-down doesn't seem to match, but then there is no oil sloshing around on the leak-down. I think you need to pull that piston out and check it out very carefully......

[CraiginTX]

What, exactly, is the cam swap? I doubt it has anything to do with the problem, but I would like to rulle this out. Are the plugs coming out of #4 dry and powdery(carbon) or oily? If you put a new plug in, does it run ok for a while? THe compression check bothers me for a rebuild, but the leak-down doesn't seem to match, but then there is no oil sloshing around on the leak-down. I think you need to pull that piston out and check it out very carefully......

The exhaust cam is higher-lift, longer duration, but same relative timing (between cylinders). So you can cut off the CAS drive and install an exhaust cam in place of the intake cam. I had a hole drilled in the cam gear (for the "pin" on the camshaft) to re-orient the cam relative to the timing marks on the gear. Several people on Miata.net claim to have dyno verified 8-10 hp gain from the swap. With the timing marks on the gears lined up properly, the lobes on the camshaft look to be in the right place. If that were the problem, I don't think it would be just one cylinder...

The plug is dry - looks like carbon. But installing a new plug doesn't help.

I agree about the compression.

What would I be looking for if I pulled the piston?

[MiataMike]

I had a similar problem. My car would run better after it warmed up. but when it was cold it ran like Ars$. it turned out the woodruff key was broken and it was causing the timing to be off 10-20 degrees.

Just a thought.

[channelmaniac]

Couple of things to check:

Double check that TDC on cylinder #1 is really TDC according to the timing marks on the crank pulley. If the crank pulley is bad your timing will be way off.

Disassemble the coil packs from the bracket and swap 'em around. Maybe one of the two coils is bad.

RJ

[CraiginTX]

Disassemble the coil packs from the bracket and swap 'em around. Maybe one of the two coils is bad.

RJ

But then cyl #1 wouldn't run right, either.

[MadMerv]

I don't really know what to look for, but it has to be something in that hole. Any oil-in-water or water-in-oil? It doesn't really sound like a head gasket. Maybe there's a stuck valve or debris keeping one from closing all the way. I'm guessiing with all these suggestions, because of how it all happened. It might be better to pull the head and check it before pulling the bottom end. Since you would have to pull the intake and exhaust manifold anyway, you could stick your fingers in and see if all valves are operating normally before removing the head completely.

[channelmaniac]

But then cyl #1 wouldn't run right, either.

Uh... yeah...

The point is though that if you swapped the coils around in the coil pack and the problem moved to #1/2 then you'd know you had a bad coil pack and not a valve train issue or some other issue...

RJ

[black roadster]

Congrats to Craig for getting her running. He showed up last night with a strong running engine.

I'll let him fill you guys in on the details.

[Ataim]

I'll let him fill you guys in on the details.

WELL?????????

[CraiginTX]

So the solution turned out to be the (not very) obvious: re-torque the cam bolts.

Mr. Day and I met yesterday afternoon to try some additional troubleshooting. We put a NOID light on the injector connectors and not surprisingly, they were all firing. We decided to pull the valve cover and see what we could see.

Everything looked fine. Pressing on the lifters showed none collapsed or anything. No broken valve springs or anything obviously wrong. We re-checked the cam timing and it looked spot on. John decided to put a socket on the cam bearing cap bolts "just to make sure" and we found that the bolts for the 2 rear-most exhaust cam bearing caps turned a bit before they snugged up. Mind you, they weren't loose before, but they did turn a little.

We ended up loosening all 20-odd cam bolts and torquing them to spec with a torque wrench in the proper sequence. I guess they never got fully torqued after the lifter replacement...

Once we got it all put back together, I took it for a spin. Smooth idle, strong power, we finally seem to be in business. I reset the idle and adjusted the timing to 14 deg. and I'm now VERY pleased with the motor.

It's only got a 100 miles or so on it since the rebuild, but it's fresh from top to bottom with a 5-angle valve job, newly cleaned injectors, the exhintake cam swap, a new 4-2-1 header and high-flow cat. Obviously the butt-dyno can lie - particularly when you've just spent many hours and mucho dinero - but it really does feel noticeably stronger, with a better pull in the upper RPMs. Once it's loosened up a little more, I'd like to get it on a dyno and see what's up.

Thanks to channelmaniac for the loan of the hoist, nealb for the replacement OP sender, trackdog for the hours of troubleshooting, and Paul ('91 BRG) for many hours help on the engine pull and replacement.

It lives...

[Ataim]

Paul ('91 BRG) for many hours help on the engine pull and replacement.

It lives...

I love getting credit for something that I had nothing to do with!!!

Paul (I had a '91 brg)

[channelmaniac]

WOOHOO!

Glad to hear it's all back together and running!

Which bearing was it that was eaten up and causing the funky sounds in the motor?

That's three cars now that live again thanks to the engine hoist.

I'm glad you got good use out of it. It's one of those tools that you just HAVE to have to do engine work like that and then it sits around for years doing nothing. I'd rather see it being used than be stingy and let it collect dust.

[CraiginTX]

WOOHOO!

Glad to hear it's all back together and running!

Which bearing was it that was eaten up and causing the funky sounds in the motor?

#1 & #2 rod bearings were pretty bad & the #1 & #2 piston skirts were scuffed, too. Stupid oil pump!

John said that while catastrophic failure wasn't imminent - it was definitely time to fix it before it broke for good...

[MadMerv]

I can't believe that a couple of "slightly loose" cam bolts could do that. "A bunch loose" I could believe.............

[Bacon]

Glad you guys figured out the problem before you decided to take your car to a farm and put it out of it's misery. I would, however, suggest that the engine's "loosened" up already! And that you tightened it down! Har Har!!

Bad joke.