Old guy with an old truck

I've got a 1998 tacoma xtra cab, 5spd, V6-3.4 with 305,000 miles. It has been a great truck, but now it is stumbling when I get to 4000 rpm, in any gear. I have replaced everything I can think of including; fuel pump, fuel filter, fuel pressure regulator, ignition control module, spark plugs, spark plug wires, ignition coils, catalytic converter, and yet the stumbling continues. The truck passes California emissions testing and performs well up to 80 mph. it starts and runs well. I have never run into fuel injector issues, could this be the problem? What else should I be considering?

 

You might try cleaning the mass air flow sensor. Two screws and one plug on the intake between the air filter box and the throttle body. Be sure and use mass air flow cleaner, not carb cleaner.I don’t know why but they say don’t use carb cleaner. Have you checked the air filter? Good luck

 

MAF sensor...

 

Waiting for the outcome...

 

Air filter has been replaced, live date shows Maf values double with double rpm.

 

I'm thinking check your electrical......battery and ground....

I run a ground strap from neg post to my head/block instead of depending on a sheet metal body ground....

There's a lot sucking on that ground......plugs.....coils.....injectors.....all at the same time.....

these trucks are getting old....and have old related problems....

 

looks like a trip to autozone in the morning for MAF cleaner.

 

300 thousand plus .

Have you checked the timing belt at all in recent times?

Ever checked the valve clearance ?? Maybe a Leak down test?

Since no codes seem to have tripped i would look at these areas.

Though adding all these new parts adds unknowns to the issue.

Good luck!

 

Cleaning your MAF won't hurt. If you're still having issues, try unplugging it to see if that fixes it. The truck should revert back to preset values for air/fuel. If that solves your issue, you know you need a new MAF sensor. Have you also looked into O2 sensors? I'm assuming you have no codes showing.

 

Thanks everyone for your great suggestions.

The air filter was replaced recently with all the other parts. The timing belt was replaced 2 years, and 4000 miles ago. This truck is used mostly for transporting kayaks to local rivers and lakes, so it does not see a lot of miles. Valves were adjusted within the year.

I sprayed the MAF sensor with CRC MAF Cleaner. It did not seem to help, might have made it worse. I tried driving it with the MAF unpluged and it would barely run. Ordered a new MAF from Amazon, should be here Tuesday.

The truck starts easy, idles smooth, and runs very strong. I drove it on the freeway yesterday at about 80mph before it started coughing. I have run SeaFoam into the intake and in the gas tank, no difference. When I replaced the fuel pump, I checked the tank for any debris. The truck has been in California its whole life, so rust is not an issue.

I will report back after I install the new MAF Wednesday.

 

Cleaning the MAF did not help, so I replaced it. My smog guy suggested it might be a "lazy" O2 sensor. It is original to the truck but not throwing any codes. What is one more part?

New upstream sensor in hand, I have the old one out but can't seem to find the connector. I have traced the wiring to the top of the 5 speed but then it gets lost. It gets very tight up there, I have pulled the heat shield from the catalytic but still not enough space to use a mirror. The old O2 and new O2 are both 4 wire, and all the connectors I can see are 2wire. If I pull the console will that give me a better view of what is going on?

 

Yes

What values are you referrin to here? Fuel trims? Long term and short term?

 

My scan tool (autel Maxicheck) has an entry under live data for MAF. It reads lbs/sec. It provided a reading, and that reading corralated to the RPM. When the RPM doubled, so did the lbs/sec. Both good signs but substitution confirmed it was working properly.

The O2 sensor is 25 years old and from what I read, overdue for replacement. It should be a simple 30 minute job. Trying to locate the connector has turned into a project with the tight clearences. Mirrors and tracing with the fingers are not sucessful at this point. My plan is to remove the console and try to visually trace the wires, failing that I will try the endoscope. 4wd and a V-6 don't seem to help access.

 

The plug for the upstream O2 sensor is on the top of the transmission and is a pain to get to. I had my mechanic replace it when I put another engine in mine. Heard you can pull the console but you still have to fish for it. Pulling the console is easy. The downstream O2 sensor plug is under the passenger seat on mine. Just pull the door sill and lift the carpet up. Good luck getting the nuts off the studs on the sensor. I bought a new pipe from Walker and just replaced that section of the exhaust. Gasket 31374. I replace mine with Denso sensors. New engine at 405,000 miles, got 436,000 on it now. Still going strong.

 

Thats what I wanted to hear, plug is in a weird position on top of the transmission. Console is half way out, will finish in morning. Just could not figure what I was overlooking. Thanks...

 

What scan tool do you have? If it is one that someone on the board has used we might be able to collect some data. You have certainly spent a small fortune on parts. At this point in the diagnosis all I can think of is if you can continuously repeat the stumbling until the PCM either sets a pending code or an actual code that could point us in the right direction.

Unfortunately the programming logic on the older trucks was still in its early days and it might take some patience to get the fault setting parameters to actually set. I have had vehicles that have had a hard miss and the misfire counter shows 0, switch scan tools and still 0, switch to a 3rd, still 0. The fact that it will miss at 4k and not 3k is odd. I am leaning towards and input sensor such as a crankshaft position sensor.

One simple thing to check is the ground wires for the PCM. They are on the upper intake manifold - drivers side, they are brown and have a tendency to get damaged. Take the fastener off and bend each wire slightly where it enters the ring terminal. It could be broken internally.

 

My scan tool is the fairly popular Autel Maxicheck. It will measure dozens of things that I can not identify, it has live data. One and Two point graphing, and a bunch of other stuff. Is there a specific parameter I should be looking for?

I can feel a small delay between asking for power and the engine cutting out. Maybe a couple of seconds where the engine will go to 5k and then stumble, like all cylinders have shut down. Releasing the accelerator pedal for 4 or 5 seconds brings the engine back to life. Holding WOT puts a lot of strain on the drive train as the engine goes from full power to off and back, several times per second. It is like a small reservior of fuel is being depleted and takes a bit to refill.

Changing so many parts, I have become very firmiliar with the brown ground wire on the intake manifold. I will give it a detailed inspection and check its resistance, once I get the upstream O2 sensor changed out. If the connector is on top of the transmission, the guys at Toyota must have a better way of getting to it.

 

The O2 sensor has a bolt holding the harness on the transmission.

I removed that bolt and secured the harness to make the connector easier to reach.

 

A specific parameter to check, no… diagnosing with the scan tool when there is a fault is great, when there isnt, it is a bit more complicated. You are going to have to eliminate the good to find out what is bad and hope that it is a parameter that can be checked. I would start off with the basics like is it staying in closed loop mode, is the battery voltage constant, is the coolant temp constant as reported by the PCM. Things like the crankshaft position sensor… while they can be ohmed a dynamic test is such a pain.

You replace the catalytic convertor, why? Did it break apart and maybe the bits have plugged the muffler to the point where it can pass exhaust until it gets loaded.

 

It turns out that the replacement of the catalytic was a waste of money. The old cat was clean, I will keep it as a spare in case this one is ever stolen. While it was being replaced I had them do a back pressure check on the muffler and it passed. Todays effort will be focused on replacing the upstream O2 sensor. I will start by unbolting the wiring harness from the top of the transmission.