Thank you for opening this thead and reading it, i'm stuck in St. Louis until this is fixed.

I look forward to your input!

I have built a 1991 non-turbo engine, and installed it in a 5 speed 1991 convertible.

Below, I have provided all the information that i can POSSIBLY think of that you would need to begin thinking about this.

All is perfect while driving, except...

It has a bumpy, random, misfire to the idle.
with no accessories on.

Also, it has a hard time keeping the car idling with the A/C, headlight. steering wheel is turned, or any combo of the above.

The idle will stabilize if you turn on things slowly.
but if you crank the steering wildly, with teh a/c on, it dies.

Also, when the idle stabilizes with the A/C on, it will stop misfiring.

It has a very delicate idle, but it will idle at 750 RPM if you do things gently and slowly.


I have checked the compression.
perfect, all faces, both rotors, all faces bump the same.
(90+ PSI)

The neutral switch seems to be fine.
on cold start, shifting from NEUTRAL to any gear alter the idle speed up and down as it should.
(so the neutral switch is okay....)

I have checked the BAC.
12v+ opens it all the way.
My scope says the duty cycle increases with the A/C coming on.
(Generally, it's fine and it kicks in at the right times.)
Disconnecting the BAC while the engine is idling, has NO EFFECT on the idle. (It should decrease the idle with the test wire grounded)
Applying +12v while the engine is idling, and the connector is off, races the engine to 3k RPM. (normal "cold start" right)

I have checked all the thermo sensors (water/air).
they are all good.

I have checked for VAC leaks.
(including re-gasketing the intake system and ACV)
There are no leaks that I can find.
(propane has no effect on the engine, even if I shoot it down the intake.) -odd-
I even pulled the vaccum line for the cruise control and it increases the idle (s l i g h t l y).


I have adjusted the TPS, also removed it and checked it on a meter, it is working and is in the resistance band that is right for a 1991 car.

I have adjusted the idle, its DEAD ON.

I have adjusted the timing, its DEAD ON.

The timing advance seems to kick in when the a/c is on and fires advanced as is should.. (it should, right?!)

I can't shake the idea that there is a mixture problem somewhere.

please, please, PLEASE,

Someone help me diagnose this one.
I'm in St. Louis to fix this car, and I cant get rid of the misfire problem!

 

Check the cabling to the BAC. I had a problem on my TII where idle didn't change when the BAC connector was removed but it clicked fine - turned out the pins in the connector had slid down in the housing and were not contacting the BAC itself. Slid them back up, epoxied in place and all was well. To be sure you can measure the resistance from the ECU end of the cable.

Also, check that you've removed the intial set jumper.

Henrik
87TII

 

I have removed the jumper.

I think the BAC connector is okay, i can hear the BAC CLICK but i will recheck this theory.

its a good one.

 

When you checked for vac leaks, did you use a gauge? if so was the reading steady? and at what?

Also does it have the same problem in the cold start cycle? Does engine temp or external temp affect anything?

 

Never seen a 91, but is it possible the two small vac lines on the front of the throttle body are swapped. On a 87 there are three vac hose on the front of the throttle body, of which only one really pulls a vacuum. The lower one(vacuum for the solenoids). If its swapped with the one directly above it (injector bleed air source), and erratic idle will occur. I have no idea if a 91 has the same setup. Timing advances with the A/C on? I'll try that today. Had no idea that happened.

 

The BAC connector is fine, no slipping contacts there.

The 1991 series 5 cars dont have the vaccum lines on the font, they are on the back.

They are not swapped.

what should the standard VACCUM reading be for a 1991 NA engine?

I cant find it anyewhere.

It bounces -v e r y- slightly, i dont remember the numbers.

I recall that it behaves the same for other new engines i have tested.

Temperature has not effect, but the cold start does not seem to be effected.

It gets worse as the idle gets closer to 750.


Irv suggested i swap the primary and secondary injectors to see if im dumping too much fuel.
Also, for $18 im putting in a new O2 Sensor today.

My scope showed VERY erratic reeadings and i know some of that is normal, but im just replaceing it to be sure.

 

I checked the timing with the air conditioning on and off and it does advance the timing about 10 plus degrees with the a/c on. I don't see it being the bac since you can see the idle change when the a/c is turned on. If in doubt stick the meters lead in the back of the bac plug and see the voltage drop. Also have no constructive ideas other than taking the relief plug off and seeing if the idle changes. That and the ole standby can of starter fluid for finding leaks.

 

Starter fluid/propame had no effect.

Removing a small vac. line (cruise control), increases the idle slightly.

I plotted the duty cycle of the BAC and it raises with the A/C operation...

I think the backfiring/stumbling at idle indicates that its running too rich. ??????

(this is a guess.)

I am about to begin the work that i mentioned above.

Keep the ideas coming folks!
it has to be SOMTHING!

 

The injectors are swapped.
the O2 sensor is now replaced.
The Mass Air Flow Meter is metering fine.
My scopemeter is out of batteries (so mo more trendplotting and metering of anything) -sigh-.

I pulled all the plugs and they are ALL black wit carbon.
No bridging of gaps, but black and sooty.

I have determined the car is rich.
I dont know why though.

When i pull a small vaccum line the car runs a "tiny smidgen" better.

but the idel is still VERY dicey.

it doesnt surge when the accessories are aff, but it does when the accessories make a big load on it.

I dont know what more to do, i have torn it down twice, moved things around, and im at a loss.

 

So what is the fuel rail pressure?? Does he have some odd combination of fuel injectors meant for some other thing? This engine broken in yet? Checked out the water thermo sensor for the ECU yet? Air intake temp sensor?

 

Hailers,

I have checked all the thermo sensors (water/air).
they are all good.

The injectors are stock.

As a matter of fact the whole car is stock.

The engine has 900 miles on it.

Its "as good as" broken in.

I dont have a spare computer here in St Louis.
If I did, I would be changing it now.

I have also noticed that one of the oil metering pump lines is foaming oil instead of pumping oil properly.

the other 3 are fine.

The OMP is going to be checked for foreign material in the OMP assembly.

At this time, i need another NA car to check values against.

ASAP.

Anyone in St Louis (near Chesterfield) please make themselves known.

I will tune your car and work on it to make it perfect so that I can compare performance and parts with this one.

You get a better running car out of the bargain!

 

Runnin' rich, eh??? Check the FPR for engine vacuum. If you haven't got it, check the PRC solenoid to make sure if it is working. It needs 12V to route vacuum (Orange connector on solenoid rack). No vacuum, 39 PSI. Vacuum, 28 PSI. You get the picture.

Another thought, replace leading plugs, old ones may be carboned up because of initial break in / timing errors. Also, maybe you are getting plug wire arcing (higher vacuums tend to need higher secondary voltage to fire a plug, thus insulation breakdown at idle is most apparent). A new set of NGK MSW wiring would dress up the engine compartment (It's a bright orange color!).
Good luck!

Irv, Keith's dad

 

Yep, listen to IRV, and check the Fuel Pressure! I have had the same problem, but on a turbo model. Some autoparts store is really happy about now

TII's have an adjustment for idle mixture, but I am not sure about your 91 NA. Mine is on the passenger side strut tower, below the MAP/Boost sensor near the rear of the airbox. Not sure if an NA has this or not.

Hope that helps....

 

I'm with Irv, Keith's dad on this one.
My N/A was doing the same thing until I replaced all the plugs and plug wires. You guys need to think small and work your way up to the bigger things or you'll waste your money. Do the stuff that gives out first. Its usually the cheapest thing on the car anyways. If the plugs and wires don't work, Check your igniters. Thats a thought. If you really want to know if your running rich or not just take a wif of your exaust and if you smell gas instead of oilly exaust compared to a piston engine, your rich. This can also though off your timing if your car sounds like a helicopter. Thats when your plugs are not firing. Play doctor. Start small.
Charles

 

Sorry I forgot.
Take a timing light and hook it up to all your plug wires and make sure they fire all the time and at the same speed. Trailing might be slightly slower than leading but that happens. Mine did it all the time. if the light doesn't fire all the time you need new wires or igniters. Its safe to get both.
Charles

 

Check out your idle mix screw on the passenger side shock tower. Make sure that the pot is working properly by testing with a voltmeter... then adjust it to the optimal setting. I had this happen to my car, and it was running too rich at idle. Also, the idle set screw on top of the throttle body... try unscrewing that 8mm nut, and adjusting that screw. Also helped in my case.

 

Ill check the ignition thing, but im gonna need to re-read irvs post to full understand it.

I know what a FPR is , but whats a "PRC solenoid"?

Do you think that a local shop will have plug wires?

 

Hey Sniper X,

PRC solenoid - Mazda's name for the Pressure Regulator Control solenoid, the solenoid that controls whether or not engine vacuum is ported to the FPR. Yep, NGK suppression wires are available at NOPI. One other thing to check, while I'm thinking of it, maybe you have a leak past your EGR solenoid - wait a minute, I can't find one in the factory manual. Make sure you still have a check valve on the purge line to the inlet duct, and make sure both that valve and the split air check valve are working, and all connections are tight. Check for hairline cracks in the intake duct. Make sure the purge valve is working (mine had a bad diaphragm). Well, that's all for now. Good luck!

Irv, Keith's dad

 

Sniper, absolutely do this, and replace the spark plug leads as well. I had similar symptoms to you and replacing just one of the leading plug leads totally fixed the problem. My car now idles smoothly with the A/C on or off and pulls smoothly all the way!

 

I have wire brushed the spark plugs clean, and noticed that one was damaged.

the electrode was a bit lower than the others, and the inside diameter of the surrounding ring was dented.

possible fromthe previous "object" that ate the first engine.... (irv knows of what i speak)

anyway, i cleaned them to shiney metal and the car runs better.


THE FPR and related things all are working right.

If i had the ability to do whatever i wnated to this thing, i would put in 4 new cleaned injectors and new plug wires too.

i have instructed the owner to replace the plugs ASAP and the wires shortly thereafter.



------ the car is running MUCH BETTER NOW that I did this -----

it still has a "hitch" to it and a idle stregth that leaves somthing to be desired, but the new plugs and wires might do the job.

Thanks everyone.....

If this doesnt fix it, I give up (for now)

Sniper_X

 

I'm from St. Louis and my parents live Wildwood and I have my 86 in Glendale trying to get it going. Here's the car's problem. The engine was rebuilt and had a Racing Beat Header and straight pipe on it. I swapped headers and stock exaust to try and get it legal. It was running fine but when I swapped mufflers it did something very weird. It would start for about 45 seconds then die. Then start for 20 seconds and die and then 10 sec and die, 5 sec and die, and then just fire and die and fire and die. Only thing I touched was the headers, muffler and actuaters to get the header off.

Changed fuel filter, added injector cleaner, checked plugs and they are new but are dark at the ends. Disconnected the muffler so its not a clogged muffler. Could it be a sensor, injector, or vacumm line. It runs great but does the same sequence of starts and dies for the last few weekends. Don't know where to start.

Any help is appreciated.

 

I realize that this is far from high-end on the tech suggestion scale, but has the ECU been checked for broken solder points, etc? That could also be playing a part...

 

This sounds like a vacuum line or the AFM harness fell off the AFM.