Mazda World Forum banner

1991 B2200 EFI, high idle speed and excessive fuel consumption

654 Views 26 Replies 3 Participants Last post by  Bobmo
Hi, first I'd like to say thanks for any advice.

I have a 1991 Mazda b2200, EFI, manual transmission with 256k miles

I have changed:
- spark plugs - distributor cap and rotor - coil - plug wires - ignition module - fuel filter - fuel pump and strainer - water pump - timing belt - rear end swap -

I have cleaned the Mass Air flow sensor, throttle body butterfly valve, Idle Air Control passages(not the entire unit), Air intake Temp sensor. Used Seafoam intake cleaner while the engine was running, Lucas in tank fuel injector cleaner/conditioner occasionally.
Achieved a much smoother idle after

The truck has seen better days, but still runs strong. Trying to fix these issues before it gets any worse or prematurely wear more of the engine.

Blows blue and black smoke on start up, idles high and gets 10MPG lately. Occasionally misses a cylinder on start up.

Unplugging the IAC drops the rpm, unsure if this indicates it working correctly or not

I plan on doing valve guides, and cylinder compression tests in the near future to address the excessive blue smoking problem.

What could be causing the loss in mileage?
What could be causing the high idle?

What are the correct ohm readings for things like the IAC, air temperature sensor, barometric pressure sensor?

Any help or advice will be appreciated
1 - 20 of 27 Posts
Welcome to the forum!
Just some advice on the smoking at startup.....I have rebuilt a few of these engines, the smoke is caused from the gunked up oil control rings on the pistons......valve stem seals won't help much at all. The oil rings get stuck in their ring grooves until the engine temp gets warmed up and the gunk on the rings & grooves heats up and the rings expand and start controlling the oil along the cylinder walls again.

On the fuel usage.....these trucks have a known issue with their ECU's where 2 capacitors leak acid onto the ECU board and the acid etches the copper traces to the point where they will corrode and cause a open circuit. The circuits that get affected are the fuel injector circuits......if your ECU has not been inspected for corrosion, I would do that first.

Keep us posted on your progress, and post some pictures of the truck if you can.
  • Like
Reactions: 1
To give you an idea of what some of these pistons & rings look like when removed from a smoking Mazda engine, upon startup, here are some pics I took a while back.

Wood Jewellery Font Fashion accessory Metal

You can see that the oil control rings on each side of the oil ring, have not extended out past the piston surface as they would normally, if they were not "stuck" in the ring groove.

Wood Metal Font Nickel Jewellery

Wood Body jewelry Jewellery Natural material Metal
See less See more
  • Like
Reactions: 1
Thanks for your reply Earl,

I checked the eletrical board box unit under the driver's seat, it looks very clean but has minor cracks in the board bottom corner. I couldnt find online where the ECU is located, is this under the seat unit the ECU?
I'll include pictures i took of that board

Compression tests went good, no abnormal/ low cylinder pressures.

I have a mechanic asking through me, have you had any experience with running an oil crankcase cleaner for a few miles to clean the gunk out of the engine, such as seafoam or "the old ATF trick" without going through a total engine rebuild? We're planning on it in the future but not currently ready for it.

Thanks again
My little work horse and all its glory.
Wheel Tire Automotive parking light Sky Vehicle
Land vehicle Wheel Tire Car Vehicle
See less See more
Hi Jay,
The ECU is behind the drivers side kick panel.
and The seafoam soak works as a temporary fix.
But It will be back.
Enjoy the ride!
Passenger side kick panel......not the driver's side!

JayCB......the unit that you looked at is the Anti-Lock brake (rear differential only) computer.......not the Engine Control Unit (ECU)
  • Like
Reactions: 1
On the crankcase cleaner.......more than likely, the crankcase itself will not be gunked up......the "gunk" that I speak of would only be partially cleaned by performing an "Upper Cylinder Cleaning" procedure. There are several products that you can use to do this, and just follow the directions on the product. But to totally clean the oil ring piston grooves & rings, it usually requires pulling the engine apart.......but doing the upper cylinder cleaning may definitely help reduce some of the smoking on may need to do the cleaning several times, but it should help loosen the crude in the ring areas. Let us know how it goes.......I will probably be doing this on one of my trucks soon also......she's got 136,000 or so on her now.
  • Like
Reactions: 1
Thanks for your direction, we'll see how it works out with some more treatment.

The truck has been a great running vehicle, repairable as it throws anything down.
I bought it for $100 bucks and threw a rear end in it, been my daily gas saver since. Picked it up half apart, seized rear end. Fun little pickup to drive hills/ rural areas and jobsite running.
Bonus pic when i dropped the fuel tank, before i broke my phone camera on that lug stud re installing it.
Just pulled ECU and checked, just as Earl said, corrosion below two little capacitors on the board.

Where do you recommend finding a refurbished/rebuilt one? Ebay seems full of pitfalls with units ranging $30-$300, if your aware of a reputable source I'll consider heading that route.
Other forum posts mention Avpro ECM, any reference on their work?
Thanks -JCB
Hi Jay,
The ECU is behind the drivers side kick panel.
and The seafoam soak works as a temporary fix.
But It will be back.
Enjoy the ride!
I knew that!
Passenger side kick panel......not the driver's side!

JayCB......the unit that you looked at is the Anti-Lock brake (rear differential only) computer.......not the Engine Control Unit (ECU)
I knew that! Sorry for the wrong direction. (thanks Earl)
For the seafoam you need to pour it in the intake till it stalls. then let it sit overnight.
or pull the plugs after the engine is warm and pour some in the cylinders and let it sit overnight.
crank the engine several times to blow out the seafoam before you put the plugs back in.
Adding to the oil wont fix your problem, but is good preventative for the next time.
I can't really tell with the blurry pic, but your ECU doesn't look too bad......definitely worth fixing! Avpro is the most reputable company that I've seen others use.....I have a couple of ECU's that were hacked when someone tried to fix them, but I don't believe it was Avpro working on them!
Yeah the board looks good, besides two little blue raspberry acid puddles right there on the board. Pictures are taken with the face cam on an old phone, sorry about the potato quality.

From what i see on Ebay, av pro might be worth the price.
Considering buying a cheap one off Ebay to plug and play, compare if it runs with that unit. Send the original to get rebuilt as well.

Thanks a bunch guys for the advice, I'll be back to give updates on its performance.
Tomorrow I get my new refurbished ECU from Rock Auto, I ended up going that route as it comes with an unlimited miles, 1yr warranty from Blue Streak Electronics.
Fingers crossed
I picked up a used engine from a truck slated for the crusher, currently need to assemble it and test it.
Forgot to grab the ecu out of It for later as well, need to go back and grab It. It's a carb, manual Trans. wiring on this was unusable, Unfortunate but it's worth holding onto if it works. Maybe a Performance rebuild if I get lucky sometime to go that route
The Carb ECU is different from the EFI.
I'm not installing the carb ecu into my truck, the rock Auto rebuild came today. Gotta charge a battery, and later I can test the truck.

First I gotta get the carb engine assembled and wired to run. Then once over it and decide if I want to add some horsepower. (Heard a mild cam, 2" pipe and weber carb is the most power It can reasonably use. More than I need)
Need the ecu to get that swapped if I want to drop it in my truck someday.
Figuring on a new wiring harness, accessories ect.

I plan on a new 2" exhaust sometime to let it breath a little more. This exhaust is original and in need of replacement.
1 - 20 of 27 Posts