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Miata = constant problem


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I'm at a complete loss and need the help of the internet....

 

1990 Miata 1.6.  At first, was idling like crap and would bog, burp, fart and crap the bed when attempting rev.  Would misfire like crazy when driving.  Caused us to leave AMP after the track day.  So what has been done to fix this and make Barber?  Glad you asked....

 

Engine was rebuilt after discovering an oil ring broke (huge thanks to Hastings for putting 3mm rings in a box labeled 4mm and us not realizing it, which explained the constant oil consumption)

New PCV with engine rebuild

New plugs

New wires

New fuel pressure regulator

Old fuel tank replaced with clean one

New fuel filter

New fuel pump

Replaced throttle body and all attached sensors with known good replacement

Replaced igniter with known good one

Replaced ECU with known good one

Replaced AFM with known good one (we have quite the collection of those now)

(Known good parts came from a local Miata parts seller that previously raced spec miata)

 

On top of all that....

 

Coil is relatively new and is working fine

CAS was previously replaced with known good one

 

Car spent a week and a half at a local shop, where the fuel tank, pump and filter were replaced and it came home with the same problem it had when it got there.  At the moment, car will idle fine.  Earlier in the day was idling at either 1100 or 1700, going directly from one to the other for no real reason.  Idle adjustment did absolutely nothing.  After throttle body and igniter were replaced, that stopped and it now idles smooth at 900.  However, easing the revs up to about 2000 and the misfiring starts.  Driving and easing the throttle also causes misfires and car to bog.  Problems seemed to sort itself out until it sat and cooled for 1.5 - 2 hours, then the same misfiring / bogging / crapping its pants started happening again.  When revs are eased through the misfire, it revs and runs fine at the higher rpm.  Seems the problem is mainly in the 2000 rpm range.  For all the internet is worth, I need its help because I'm completely out of ideas and pretty well out of parts to throw at it.  If ANYONE has ANY ideas, PLEASE share them

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So, it seems temp dependent.

 

Coolant temp sensor? 

 

Throttle position sensor?

 

Just because the parts are "known good" doesnt mean they are.

 

There should be test procedures in the FSM for each sensor.  Start looking at ecu inputs and outputs and testing sensors.

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I would start from scratch. Assume nothing is “good”.  Check and verify everything. Timing belt set correctly, good compression, fuel pressure correct, check for vacuum leaks(likely cause of high idle), timing check, etc.  

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A few thoughts for you as it sounds like a vacuum leak but you can easily test that stuff. Get a spray can of ether and with a tube nozzle with motor at idle start spraying each item and listen for motor speed change. Easy to do and mainly spray around each injector as o rings leak easily. You can spray around anything which might be a leak.

 

If that finds nothing I would find a confirmed good CAS as they are know issues on Miata's. Take an old fashioned timing light and check each plug for solid and proper fire while running motor in the range of the miss. I am assuming you are setting idle with the air screw on the throttle body? Really not much for sensors that do much with the ecu, water temp and fan switch so don't spend much time looking there. 

 

If you have the timing light check to timing and remember to add the jumper in the test plug, we ran about 14 degrees at idle. Rev the motor up and watch it advance.

If you rebuilt the motor might pull the front covers and check cam timing and belt but I don't think that's the issue.

 

Hope you find the problem and we see you at Barber and swing by as we can talk about an ecotec swap 🙂

 

Edited by 55mini
spacing
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Timing has been checked and reset, fuel pressure checked. checked for vacuum leaks, timing chain checked, good compression.  Going to pick up a temperature sensor later today and use the timing light jumper on each plug to see what they're doing in that range (I like that idea).  I know its going to be something stupid.  It always is

 

Thanks for the suggestions.  I'll start checking things off and see what happens.

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If you have temp gun shoot the header a see if you have a cold cylinder.    We had some head scratching symptoms a few years ago that tuned out to be a camshaft that had broken.   Just don't over look anything, with the thought "oh it can't be that"

 

 

 

Edited by JDChristianson
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Are you missing while its idling?  Can you unplug spark plug wires while running and see if a cylinder is dead?

We recently had an injector fail, which was a first for us.  It has somewhat similar behavior to what you were describing until it died completely and we could tell we had a dead cylinder. 

You can check for sure by finding the bad cylinder, removing the plug wire for that cylinder and see if the behavior is the same.

 

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New spark plugs went in at the rebuild I assume? Gapped correctly? Nothing fancy/standard copper units?

 

We had a weird misfire we chased for a few events on an old Mini. New plugs tended to help, but would miss under load around 5k till around 6k then it would clean up and run just fine. Also got better as it warmed up. Checked for intake leaks, played with the carb jetting, bad fuel, fuel pump, etc. Finally found out that we were running the wrong plug gap for the compression ratio/fuel we were running.....

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Probably not applicable to a 30 year old Mazda, but I have heard of the incorrect type of plug doing weird stuff to 90s/2000s Honda's, even thought the plug manufacturer said it was intended for that application.  I've heard of bad alternators/voltage regulators causing issues too.

 

But I agree with the other comments - start with the basics and assume nothing is good, even brand new or reman stuff.

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Agree with the others about vacuum leaks. I had a port on the bottom of the 1.6 manifold that wasn’t plugged that caused me some issue.  Be sure to spray all around the intake.

 

The AFM could cause the issue, but if you’ve had multiple on the car, probably not it.  
 

If you don’t have a thermal gun to check temps for each exhaust port, can spray water on each and look for it to evaporate.  
 

The other potential problem could be a broken wire to a sensor or ECU.  Hard to track that down, but it can be done.  Good news is they are “dumb” ECUs so not a ton of sensors to check.  

 

Assuming your at Barber, I’ve got a spare PCM/CAS you could try.  I know both are good as they came off the car.

 

Side note, still running 1.6 brakes?  I have a set of new rotors that I can’t use now (1.8 upgrade).  Free if your interested in them, I can bring them to Barber.  I think I have a set of new ST43 front pads also (not free).  Send me a PM if your wanting to meet up.

 

Todd

#341 On Second Thought Racing

NA Black with White Stripe/Hard Top

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So after replacing the temp sensor on the back of the block yesterday with no change, I fell back on the time tested method of staring blankly at it and wishing it would fix itself.  While performing this intricate procedure, I finally heard a sucking noise under the manifold which I THINK is a leak from where its joined to the head.  Going to spray some carb cleaner around it and eventually pull it off this morning and redo it.

 

In response to the other posts, plugs are new and all firing as they should (tested that first), no cold cylinders.  It idles fine, just misfires like crap once it revs to about 2k.  Yesterday, it would chug and misfire until it nearly died when I would bring the revs up.  Hoping this new and exciting sucking noise is causing a vacuum leak and causing this problem so we can get it fixed today.  Looking forward to heading to Barber and bringing shame to the family name again!

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The fuel is the trick. You have extra air coming in so the fuel be it carb/brake cleaner or propane will help you pinpoint the location of the leak. If you added the fuel in the air box you would notice the revs increase with the extra fuel added with the vacuum leak. Seems you are close to the solution. 

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Does that car have an EGR valve that is still active?  Had a '99 that would go all wonky from time to time due to the ERG not being where it should be because it was sticky. 

 

For vacuum leaks a cheap hand pump and a couple of cheap cigars will allow you to pump smoke into the intake and see where it leaks out.  Can be easier to see what is going on when you don't have fans blowing and belts trying to grab at your sleeves. 

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I didn't read all the responses, so this may have been suggested.  Is the CAS out 180 degrees?  It will fit in two ways but only one is proper.  We put it in 180 out one time and it would run but not rev or pull under load.

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  • 2 weeks later...
On 12/7/2020 at 4:06 PM, Hanz and Franz said:

 a smoke machine

 

+1 for a smoke machine for vacuum leaks.  I recently pieced one of these together with an old paint can, some air hose, a heating coil and a 12v battery. There are lots of how-to's online for building one.

I was pretty sure I had a vacuum leak, tried carb cleaner to find it and the idle was not stable enough to really tell a difference.

Hooked the smoke machine up to it and found the issue in a matter of minutes. 

I used baby oil as the "smoke" additive, which does stink when burned but it produced some pretty thick smoke.

 

At a minimum if you do this test you can be absolutely sure its not a vacuum leak.   Another good use is for finding pinholes in welds for various things that should be sealed.

If you are in the Raleigh, NC area you are welcome to come borrow my kit.

 

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  • 4 months later...

For the 3 people that may find this interesting.  After all the parts replaced and threats of physical harm made toward the car, the problem turned out to be................ injector harness.  Car took a dump after 8 hours at Road Atlanta after having some work done by Primal (Racing Analytics) to fix the original problem that started this thread.  After a return trip to Primal following Road Atlanta, they found the injector harness was contributing to injector failure.  So our consistent, time delayed 6-8 hour misfire that caused us to drop out of all but 2 races turned out to be that stupid harness.  Had it replaced and finished Daytona with no issues aside from the exhaust falling off from the header (which a trip to home depot fixed).  So, huge thanks to Primal for finally figuring this out and getting it fixed!!

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So I'll be curious who the other two people are.

 

Was the harness problem just a function of it being a 30 year old harness that has probably been plugged and unplugged a lot of times?  Or was it something more intrusive like it got damaged through a distinct action?

 

I saw another Miata fight through a day of misfires to find it was the wiring into the plug at cam angle sensor.  The wires just got tired of being pushed around and the plug coming and going. 

 

FWIW our friends at Ballenger make most all of the plugs that you would need to do a complete new harness for the car.  It is an impressive list when you filter for the Miata.  Makes me glad to know the parts are available, but I cringe at how much time you could invest.

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55 minutes ago, MMiskoe said:

Makes me glad to know the parts are available, but I cringe at how much time you could invest.

Invest the time once, and not worry about it at the races, where time is much more valuable.

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