2.3 Low power at low speed

[the_uglydog]

One of my 96 Mazda B2300's with the 2.3 is doing something I havent run into before. No codes are showing and the fuel pressure is within specs. It has no blowby and reasonably good compression for 175000 miles. It will start and run fine, then after a bit it will suddenly stumble then sometimes recover and sometimes stall. It might run fine for 1 or 2 hours then do it again. There's no smoke coming out of the tailpipe. It does it more before it's warmed up. Also the power is way low at low rpm's, you can barely step on the gas or it will stumble and backfire, unless you pump the gas pedal. It acts like an older vehicle that has the distributor retarded, the spark advance not operating or the accelerator pump not working. Once it reaches higher rpm it runs fine. Here's what has been changed by the previous owner since this started.
New plugs... all 8, new plug wires, new cat, new timing belt (timing is right), new fuel filter, new throttle position sensor. I also changed the trans filter and fluid and the level is correct. If I unplug the MAF sensor it doesn't make much difference though, so I was wondering if it might be that even though there's no codes. Also there is no oil or deposits of any kind inside the tailpipe, plugs are not wet or fouled but they are a smooth semigloss black.
Has anyone run into this?

 

[travisl1345]

sounds like your timing belt jumped a tooth or two... you might look at the sticky that is above this post, could be the answer to your problem or maybe not i didn't see the timing belt part.... but you might still check it because his sounds very similar to what was happening to my truck

 

[the_uglydog]

That was what I thought

I thought the timing belt also. I checked it, they had just put on a new one and the marks lined up perfect......

 

[Mirage]

This is a good one. The stumbling and stall first led me to believe you have a ignition problem, as does the glossy black residue on the plugs. Perhaps you have a fuel injector sticking open. Do all 8 plugs look the same?

Maybe check the plugs and make sure hey are of the proper heat range? I've sen and heard of similar problems in older V8's (back in the day, as they say now) when people used to swap out hotter plugs thinking they would get better performance, and not really knowing what they were doing.

Next you say this problem happens more frequently when cold. I's take a look at the AIC valve. I've seen some really weird behavior caused by this sucker. Clean it really good, with throttle body safe cleaner, and replace the gasket and see if that helps.

Other than that, I don't know. But having a 96 2300 myself, I'll keep thinking about it.

Good luck!

 

[tomw]

If you didn't throw a code when you ran it without the MAF, then there is something wrong with the ECM. Did this problem of low power at low rpm come on all of a sudden or over time? Does the stumble have any relation to the low power, or is that another problem?
What ignition system does this have? Does it have a cam position sensor where the distributor used to reside, or does it depend on the crankshaft sensor? If the former, it may be mistimed.
tom

 

[Bryan22]

I'd have a look at the EGR system. If the valve is stuck open a little it will really run like crap down low.

 

[the_uglydog]

Good suggestions

Great suggestions, I'll look at those tommorrow. It didn't turn on the light so I didn't recheck the codes after unpluging the sensor so i'll do that again tommorrow. I don't know when the problem started, i just bought it. It was traded in at a Toyota dealer then went through the auction and ended up at a used car dealer. They all did all of the regular things you would suspect, couldn't fix it and gave up, now it's my turn...... Couldn't pass it up for the price, even if I spend a couple of grand fixing it i still stole it.
Ill look to see if it has the sensor where the distributor used to be and check timing if it does.
Thanks

 

[the_uglydog]

What i found so far.

When i rechecked it did show the code for the MAF sensor after unplugging it. The check engine light stoped working........ i cleaned the MAF sensor, the temp sensor and the throttle position sensor. I also cleaned the idle air valve and EGR valve. It got somewhat better after clearing the code. I checked all of the spark plugs this time, a few were wet with gas. I noticed a few of the wires came off rather easy so I checked inside, someone had pulled the metal ends off of the ones under the intake manifold, interesting since they were new from the owner before he traded the truck. ( I guess his mechanic didn't like him) It runs quite well now, normal power in 1st, but it still hesitates on takeoff and will even backfire sometimes when its not warmed up.
The throttle position sensor reads open 18* at idle and immediatly reads any changes down to 1/10th of a degree. Idle 750 rpm, up from a previous 625, when warmed up , a/c off.
Timing works off of a cam position sensor. The advance is 25* advanced at idle and drops to 7-10* advanced when reved up a little and holds at that.
Temp sensor matches temp probe stuck in the air intake, air flow varies with throttle opening and responds quickly to changes.
Fuel trim stays at about 3.9
Even when it hesitates no misses show on any one cyl, but it does show low grade random misfires on all cyl if you put a load on it. Every once in a while, a lot less often now, it will still suddenly sputter and either stall or recover.