Mazda Bravo - Manual 2x4 1994 b2600 G6 - Lagging, running rough, flat spotting

Hi everyone.

Car is a 94 Bravo b2600

Car WAS running on LPG, now running on petrol only.
All gas components have been removed.

I have replaced the following

  • distributor cap

  • leads

  • rotor button

  • plugs

  • removed and cleaned all injectors, checked ohms. All within spec

  • replaced injector seals

Timing is set to 5 BTDC using a timing light.
ECM was grounded per the manual.

ECM was replaced as the old one was toast.
It was a G630

Replaced with unit from wreckers with G18 from a Ford Courier of the same year and same motor.

No engine lights are present.

Car will idle and run without a problem.
Idle is sitting at 750rpm

It is bogging down around 1500rpm-2000rpm, then running out the rev range without a problem.
HP is compromised when driving. It has to be “feathered” to get up to speed.
Sharp stabs of the accelerator will make it stall.

I am at a total loss.
It will be something simple.

Any advice welcomed.



1 Like

OK, 28 year old vehicle and 28 year old electronics.

Electronics go bad with time because the capacitors in the electronics degrade or fail. You had a knackered ECU and replaced it with an equally old ECU from a wreckers. I’d guess that replacement ECU is also on its last legs causing your problems. A brand new-old-stock ECU would have the same problem.

The only solution may be… swap to a carbureted, non electronically controlled system OR install an aftermarket ECU and get it configured and tuned for the engine. Not great choices for a vehicle this old.

1 Like

You may need a different distributor, A company I used to work for had a lot of LPG vehicles,

The distributors were custom for them, different weights/springs and the amount of advance was different from the petrol engine…

You might consider going over the PCV hoses and the intake tract above the throttle plate and out past the MAF sensor. An air leak in those areas could cause a flat spot.

One has to wonder just how close that ECM replacement is and whether or not that could be the cause of the problem. Same year and motor may not mean much.

Connect a fuel pressure gauge and monitor the fuel pressure while driving.

I have a manual trans Corolla, similar vintage. Never experienced that problem though. It seems like you’ve done all the right things.

Did it ever run correctly after ECM change-out?

The fact that it idles ok pretty much rules out vacuum system leaks. I’m guessing, from most likely to least …

  • EGR problem of some sort
  • Fuel pump problem
  • Exhaust system is clogging, faulty cat possible
  • Throttle position sensor problem
  • Clogged fuel injectors
  • Faulty distributor
  • Engine air cleaner filter
  • Valve clearance problem

If you can figure out a way to temporarily disable the EGR valve from opening (as a test for this being EGR related), that’s where I’d start. Your EGR system is probably partly computer controlled, and new ECM may not be exaclty configured for your car. For example, on the Corolla, the Calif ECM configuration is different from the other 49 state ECM configuration.

Is the CEL on? Have you tested for stored diagnostic codes?