67 replies to this topic

[ECOBRAP]

Have you checked cam timing?

 

When I hold the rpm's at 3k the car still stutters, which I don't believe could be caused by cam timing?

 

 

If as stated all the exaust manifold tubes are the same temp then the spark plug may be an oily red herring with respect to the main issue, but still needs to be resolved. Also, I don't recall it sounding like it was running on 3 cylinders which is quite distinctive and would be obvious when you drive it.

 

When I say same temp, not exactly the same but no obvious temperature differences. And yes, the car felt fine under power tonight up in the mountains which tells me the cylinder is still firing.

[Steve Scheifler]

OK, so an injector sticking open or leaking badly fits. Had one like that on the dyno a few weeks ago. You really should avoid running it that way because it can wash down the cylinders which Is hard on the rings.

[B(Kuch)Kucera45]

Make sure when you pull the fuel rail up you don't lose any rubber o rings. When you put it back together just spray a little silicon on the o rings this will help them slide right in without slicing any of the o rings.

[RazerX]

If an injector got dropped that could do it.  I know those injectors were cleaned and balanced about a year and half ago.  

[RazerX]

Matt, 

 

I know #4 looks like the obvious path at the moment.

 

Did you verify crank timing mark was at TDC?    And did you pull the valve cover and verify timing belt was applied properly?  Given you are waiting for an injector these are easy things to cross off the possible list. 

[mjw]

It is hard to tell from the video but are the spark plug wires for cylinders 3 and 4 swapped on the coils?

 

http://www.miata.net...ges/TBELT11.JPG

[Steve Scheifler]

Wow. Good eye. But would it really drive as well as he says it does with them reversed? That seemed to rule out a lot of things, but I haven't actually tried running one like that.

[mjw]

It runs much better than you would think, I did it on a 1.8 and it would idle very similar to this and once the rpm gets higher up it smooths out.  I spent several days working on it until I realized my mistake.

[Ron Alan]

I didnt think it mattered the order as long as 2 wires are on the correct coil? Doesnt  the coil fire on compression and exhaust stroke?

[ECOBRAP]

Plug wires are in the correct orientation.

 

Replaced #4 injector, no cigar FML 

 

Taking it for a quick drive and I will check the spark plugs again and see if there is any difference. If not it may be a spark problem.

 

I will check timing today as well. Running out of options here so AFM, CAS, and ECU are pretty much the only variables left.

[David S.]

I'd have a hard time believing it is anything that wasn't changed with the new engine. Have you talked to the engine builder for his thoughts?

[mjw]

I didnt think it mattered the order as long as 2 wires are on the correct coil? Doesnt  the coil fire on compression and exhaust stroke?

Yes, 1 and 4 have to be on one coil and 2 and 3 on the other.  But if you put 1 and 3 on one, then 2 and 4 on the other, it still runs but idles poorly and then smooths out at higher rpm.

[ECOBRAP]

Went for a drive, #4 plug is still black. Not sure how long it would take to clean, but it was a 45 minute "spirited drive". Swapping plugs and going for another drive to see if that plug starts getting black too.

 

Considering the issue only seems to be on one cylinder, I think that eliminates the AFM CAS and ECU as the problem. Has to be some spark, fuel, or air problem on that cylinder.

[Bench Racer]

Coil???  Plug wires???  Crap where plug wires fit in coils???

 

Disconnect the throttle position sensor when cold and when hot for difference while at idle. IIRC same switch plug wire disconnect as the AFM.

[ECOBRAP]

I have swapped coils, plug wires, and plugs, no difference.

 

Just checked engine timing. At TDC, the #1 cam lobes are pointed to the outside of the car (9 oclock and 3 oclock). The teeth on the cam gears line up with the markings on the backing plate, but it is weird that "I" matches to "E" and vice versa. On all videos and pictures I found online, it seemed that the "I" on the cam gear should point to the "I" on the backing plate instead. I am assuming the builder just put the cam gears on wrong, but this makes no difference in cam timing so I can check that off the list.

 

I am pretty fed up at this point. I have spent two full weekends trying to diagnose this, which has taught me a lot, but my first race is in two weeks and I just want the thing running correctly. I will likely have to hit the dyno with this problem and just see what happens. I still have not taken this new motor above 5500rpm, so it is hard to tell if it is good on power.

[Steve Scheifler]

I'm not sure you can completely rule out the ECU even if it is always #4. It probably isn't the issue, but I'd still prove it. Mechanically these are very simple machines with a limited number of things that can cause your symptoms, but the electronics that control timing and fuel are complex and critical.

I've lost track, did you check compression? On the mechanical side thinking a sticky valve is a possibility.

[ECOBRAP]

Yes, compression was normal and even across the board. I may be able to steal a friend's ECU and AFM this week to try those.

[davew]

Always go back to the basics. In high school auto shop we learned you need 3 things (air,fuel, ignition) to make any engine run properly. And they need to be at the right time and with compression.

 

You state the  compression is normal. What number is normal? You never mention doing a leakdown test, nor its results.

 

You most economical option (when factoring in time, and frustration, as well as money) may be to take it to a professional. The tuition at "the school of hard knocks" can be very expensive.

 

Dave

[chris haldeman]

I would also add you seem too have ruled out the cas??? It is the only part that tells the computer the engine is spinning. It is the only communication too the ecu and sends signals for timing and FUEL .Also the very first thing too check on any na Miata

[ECOBRAP]

Always go back to the basics. In high school auto shop we learned you need 3 things (air,fuel, ignition) to make any engine run properly. And they need to be at the right time and with compression.

 

You state the  compression is normal. What number is normal? You never mention doing a leakdown test, nor its results.

 

You most economical option (when factoring in time, and frustration, as well as money) may be to take it to a professional. The tuition at "the school of hard knocks" can be very expensive.

 

Dave

 

Yep, should be able to get my hands on a friend's ECU + AFM + CAS tonight. I will try those, and if those don't work I will be dropping the car off to the engine builder who said he'd help me figure this out.

 

Thank you everyone for the suggestions and help. Autoshop class doesn't exist anymore, so for a young gun like myself this diagnostic hunt taught me a hell of a lot and I am now much more comfortable working around the engine bay.

 

I will post what the problem is as soon as I find out.

 

 

Best,

Matt