2.7 stumbles and bucks under load (solved)

About the truck:
2.7 3RZ-FE 4x4 5 speed 200k

Symptoms:
-No codes! please dont ask because I have none. I check it almost everyday with the scanner hoping to see something.
-Truck idles fine at right around 700 RPM but can still barely hear what sounds like a misfire when reving the truck
-Truck bucks and stumbles especially between 1800 - 2700 rpm in all gears
-RPMs do not appear to go down when this happens but truck bucks ( feels like it turns off and restarts but in a split second)
-symptoms become much worse and noticeable once the truck warms up.
-Also noticed truck has a hard time starting immediately after turning it off (cranks a few extra times)

Maintenance I have done so far:
-Replaced Spark plugs, wires, distributor cap, and rotor
-Replaced fuel pump and fuel filter
-Cleaned MAF
-Taken off throttle body, cleaned it and the IAC out, and replaced gaskets
-Replaced TPS
-Replaced headers and downpipe with 2001+ mini header (new gaskets all the way to cat)
-I have adjusted valve shims to spec, replaced valve cover gasket, and plug gaskets

Tests I have done:
-Ran the car with exhaust disconnected before the CAT (truck still bucked)
-Tested EGR sat, and then isolated EGR so valve remain closed (truck still bucked)
-Disconnected TPS and ran truck (truck still bucked)
-Disconnected front 02 sensor and ran truck (truck still bucked)
-Disconnected MAF sensor
*THIS HAD SOME RESULTS* truck died when first disconnected. Restarted truck without the MAF and had a definite rougher idle. However, the truck finally went into limp mode. In limp mode i had absolutely no bucking, truck ran through RPMs and shifted fine. It was definitely in limp mode though because it didn't really rev past 2500 rpm in any gear over first, and slowly came up in RPMs.


Is it right to think that a sensor is at fault if the truck runs fine in limp mode? I'm kinda running out of ideas with this thing.

SOLVED: Replaced distributor with junkyard oem denso distributor from 95' tacoma. No more bucking, no more misfires, no more hesitation. Problem fixed.

Here's some sensor data that I got while driving around with the obdII reader and torque app.
At IDLE:

At 2500 RPM:

 

If you don't have any better ideas and you can find a junkyard Denso distributor that matches, try putting that in. It may be less common on Tacomas, but corollas from the 90s sometimes have weird symptoms like this that a new used Denso distributor will fix. The aftermarket distributors can be flaky.

 

Mine was doing similar, maybe a different problem and my MAF is giving bad voltage outputs. Idled fine the other day but then was jumping around when it shouldn't have been. I had cleaned it trying to fix the problem, but it is just shot. Going to replace mine now.

 

Sounds kinda like a maf issue. That's an expensive part to just throw on though :/

I was leaning towards fuel but you already changed the pump and filter.

 

By the way, your truck looks awesome

 

Yea, fuel was my first thought, and with it being an old truck I chose to replace the pump and filter anyways. I am somewhat leaning towards the MAF as well, either the sensor itself or the wiring. I posted some sensor data from a drive I took yesterday. I have numbers at idle and at 2500 RPM. Not sure if those MAF readings are too high at 2500 RPM.

 

How long has it been doing this?

 

With the truck idling tap the mass airflow sensor lightly with the handle of a screwdriver and if the idle fluctuates the mass airflow is bad

 

I had similar symptoms in an old 2003 XTerra of mine. Turned out the dizzy bearings would go bad (or start to) and would create all sorts of hesitation under load. I do remember a rougher idle though, so MAF does sound a touch more fitting.

 

Since I bought the truck about 4 months ago. It's not my daily so I only drive it once or twice a week. I got it on the cheap. The engine fires right up, idles strong, and actually pulls pretty hard when its not stumbling. Figured I could fix this issue and a have a descent taco for less that 2 grand.

I'll give that a go.

See, I feel like with it driving so consistently in "Limp mode" with no hesitations it kind of rules out certain mechanical things. Like if the bearings were bad causing lack of spark (bucking), why wouldn't I still feel that affect when the truck is in limp mode? To my knowledge, when in Limp mode the ECM ignores the sensors and uses baseline values. With these baseline values the truck runs great with no bucking or skipping. This is what really makes me think a sensor is bad, but just not bad enough to throw a code.

 

If it were actually in real 'limp mode' you bet it would throw a code.... Since you have not-a-one the computer must think all is well. I'm inclined to assume a more "mechanical" problem...but then again, I dont know what the heck the computer really actually monitors to throw codes lol

 

Compared to my truck those fuel trims are really high, I'm thinking you have a vacuum leak and it hasnt become bad enough for the computer to complain.

 

Not so much, especially on these older trucks. My MAF has been in and out of working and it doesn't show on my cheap scan tool, a friends $500 scan tool picked it up as an active code, and then we watched the voltage output on the MAF jump around under throttle.

 

Whats going on with the AFR readings?

 

I should clarify. The truck has only gone ito limp mode when I completey disconnect the maf. And when I do this it does in fact throw a code and the check engine light illuminates. Driving regularly however with the maf connected I have no codes and the car is not in limp mode but does buck and jerk all over the place

 

I added that, thinking it would register but that's clearly not information that you can get from the obdii system.

 

In normal mode the timing is advanced when you step on the gas. The computer calculates more air and more gas going into the cylinders, so it has to advance the spark to compensate for how much longer it takes the air-fuel mixture to fully ignite. In limp mode the ignition isn't advance as aggressively at the expense of power, emissions and fuel economy.

The point is if it stumbles when you step on the gas pedal it could be fuel, air or ignition related. They all have to work together. The distributor could be producing a weak spark only when the timing is advanced. It could be a bad sensor reading throwing the computer off. It could be a vacuum leak causing more air to get in than the computer thinks.

 

So basically a spark isn't just a spark. Your saying that when more fuel and more air are added into the equation then the spark needs to be stronger. So my distributor could possibly be creating a weak spark that has no problem igniting the air/fuel mixture when in limp mode, but can't ignite the higher air/fuel mixtures that are produced during normal driving when the ECM is in charge.

DrZ since you seem to have a good grasp on limp mode vs regular driving. Does the fact that the truck runs perfectly in limp mode tell me anything at all as far as trying to diagnose this problem?

 

That and the spark is advanced more under higher load. It sparks before the piston is at the top to allow the gas enough time to be completely burned by the time the piston reaches the top to start the power stroke. I'm just saying don't rule out an ignition problem based on your symptoms. I don't like to throw parts at a problem, but if you can find a cheap used distributor to try it might be worth a shot, otherwise check the easy things that others have suggested.

It may or may not mean anything. The limp mode is trying to cover up problems.

The Toyota repair manual lists exactly what the computer does in limp (fail-safe) mode based on the code. For example with P0100 (Mass Air Flow Circuit Malfunction) it says

Ignition timing fixed at 5° BTDC.
Injection time fixed

• Starting .................. 11.6 msec.
• Closed Throttle Position switch ON ...... 3.2 msec.
• Closed Throttle Position switch OFF ...... 6.0 msec.

So, when you disconnected the MAF sensor, the ignition wasn't advanced more than 5 degrees and the amount of fuel was fixed. In normal driving these things are adjusted based on each other, so it still doesn't give you much information.

Since it runs well in limp mode it's probably not a problem with the pistons or valves (like low compression). It still seems to point to a fuel, air or ignition problem.

 

Well there's no obvious vac leaks that I can find plus the strong steady idle kind of make it seem like its not a vac leak.

Only thing left to replace in the fuel system is the fuel pressure regulator and/or the fuel injectors.

The fact that this get's worse as the truck heats up make me thing it may be something electronic that becomes more degraded with heat. I really don't know though, I started with all the easy stuff and now it's getting a little harder to narrow down.