2.3L ('83-'97) Mazda B2300 not cooperating

[kblast523]

1996 Mazda B2300 here. Probably 250k+ miles (odo not working since 2009), replaced: plugs, wires, MAF, TBS, filters, fuel pump, regulator, up and down stream O2 sensors, cat converter, TPS, PCV and hoses. Replaced upper intake gasket, thermostat (195 degree), water pump, timing belt, serp and all idlers as well as power steering pump. Replaced fuel pump and regulator last week due to codes and new pump and regulator made no change to measured fuel pressure of 28 lbs at idle. Start vehicle hot or cold, will not idle. Engine speed fluctuates up to 2000 then when idles down dies. Runs with throttle above 1000 rpm. It launches ok from stop, but has to be run at 2000 rpm or more to keep from slowing down, if it's put in 4th below 55 to 60 mph, slows down. Top speed 65-70 mph in 4th does nothing unless going down hill with tailwind and tailgate down in 5th....not using oil, no smoke, not engine knocks, no sludge. Three ASE certified mechanics stumped, as well as I. HELP!

 

[black_demon69]

Do a compression test also see if fuel pressure drops like a stone when keyed off. Might be leaky injectors. I am currently doing a 93 with bad valves so that could be the problem.

 

[kblast523]

What kind of compression do I need to see? Fuel pressure was 28 lbs with the old and the new pump and regulator. New fuel filter installed less than 200 miles ago. The engine had 2r and 3l plugs only partially screwed in and ran ok with iridium plugs burned open to .85 and five different brands of wires. The MAF had been replaced prior and had zip ties and electrical tape holding the connector inside air filter housing in place. My 88 with 1/4th the approximate miles would only run about 72-74 mph in 4th, so this didn't surprise me Runs the same with vacuum off EGR too.

 

[black_demon69]

you should see a little over 100 psi on compression test. you could also do a cylinder leak test also. 28 psi for fuel pressure is a little low what happens when you remove vacuum from the fuel pressure regulator what fuel pressure do you see then? the 2.3 only had around 100 horse power new so well not really speedy little things.....

 

[kblast523]

No change in fuel pressure when regulator unplugged. Mystery is when I replaced the valve cover gasket the top of the cylinder is clean, the cam is like new, not even much discoloration on the bottom of the cover and virtually no blow by when running. Book says minimum fuel pressure should be 35-38, pump pressure vs rail pressure is difference.

 

[RonD]

1996 2.3l will run fine with 28psi fuel pressure, but it could be dropping more at highway speeds, but you would FEEL THAT as stumbling "like its running out of gas" because it is
So fuel pressure is probably not the issue

1996 2.3l should be 150 to 160psi compression

If 130psi or less then could be timing belt issue, its stretched

Clogged exhaust
Engine is a self powered air pump
If old air can't get out then new air with fuel, can't get in
You can test with vacuum gauge
At idle engine should have 17" to 21" of vacuum
Raise rpms above 2,000 and hold it there
Watch the vacuum, if its slow dropping then exhaust is blocked, old air can't get out so new air can't get in and you lose power

 

[kblast523]

New timing belt 250 miles ago, three ASE certified mechanics have checked timing. New CAT converter, old one 25-30% blocked. Could be baffle in the FACTORY 25 year old muffler...Remember, Texas truck, sold new in Odessa (dry), lived most of its life around Comanche, Bangs, and Boerne, I bought it from a guy in Canyon.

 

[scotts90ranger]

They might have checked the timing belt alignment on the cam and crank, but did they check the auxiliary pulley? The cam sensor runs off that, and being a sequentially fired engine, it needs one of those to run right, it sounds an awful lot like the wrong timing mark was used on that pulley...

 

[RonD]

When you have a lack of power you check compression first, its a mechanical function, good or bad test, can't be intermittent or a "grey" area, good or bad
Once that is off the table you move on to spark, fuel and air flow

Very easy to test air flow with vacuum gauge

You can replace parts at random and maybe stumble upon the problem, nothing wrong with that, people do it all the time

Computers and sensors don't change the way a gasoline engine works, they make it more reliable but it still runs on the same 3 things
Compression
Fuel mixed with air
Spark at the right time

 

[kblast523]

First tech found the aux pulley 180 out. Correcting did not fix idle problem. Had better (not much) power before replacing whole raft of parts. At least it would idle, run, start fairly easy. This thing even had all the factory heater and radiator hoses. It has had the front end rebuilt, ball joints and tie rod ends have grease certs.
Didn't start with "random" parts replacement, replaced all 'service parts' that would affect reliability. Hey, I built a 1960 Corvair engine that would run circles around a Camaro in the day, and a 1966 Mustang 289 c-4 that turned a 13.7 sec quarter mile at Penwell Knights in Ector County.

 

[RonD]

Wasn't trying to be insulting, just saying if you start with compression and take that off the table you will always be better off

1996 2.3l runs 9.4:1 compression ratio(CR), seen here: https://www.therangerstation.com/tech_library/4cylinders.shtml

General rule of thumb is 18 X CR = expected PSI
18 X 9.4 = 169.2psi
But that's a new engine, so 150-160psi would be expected
As compression goes down so does power

 

[kblast523]

Ok, thanks...be looking at that Monday