2.7 stumbles and bucks under load (solved)

Hey on the bright side, at least when you get it fixed your truck will run like new since you've replaced so much. Lol

 

EGR, maybe.

 

I have tested the EGR by applying vacuum to the valve and observing the engine cut out, so I know its working. And I have driven around with the vac lines removed and plugged for the EGR so that it couldn't come open and it still stumbled. Only thing EGR it could be is maybe a vacuum leak at the EGR to intake manifold connection. I'm going to take it off and clean it out sometime soon and replace the gaskets.

I think my next order of business is to put a modified banjo bolt with a fuel test Schraeder valve and just confirm my fuel pressures are in spec.

 

Anyway to differentiate between a problem with the ignitor vs the distributor? I know cam-position from the dizzy is sent to the Ignitor, then from the Ignitor to the ECM, then spark advanced is sent from the ECm back to the Ignitor. Seems like they are both just as important to proper spark.

 

Does the problem come and go as you move the throttle around? Grab your multi meter and check out the throttle position sensor. As you go slowly from idle to full throttle, the impedance should change smoothly. If there are any jumps or sudden drops then the sensor is bad.

 

It's hard to say if it happens at a certain throttle position, but it does seem to happen between 2000-2500 RPMs. I have observed the waveform on my torque app over the OBDII and it is smooth and consistent. I haven't however tested directly with a voltmeter. I just figured if it is reading smooth at the computer then it's probably working right.

 

Went ahead and ordered a new TPS just because so many people say that even if they seem to test right they can still be bad. I have my doubts that it will fix anything. I'll just throw it on and eliminate TPS once and for all as a culprit. Waiting on a few parts (special banjo bolt and schraeder valve) to test fuel pressure.

 

When you pulled the plugs, what did they look like?

I've seen this symptom with a failed head gasket. Failed just enough to partially quench a cylinder (sometimes) yet not lose coolant etc etc.

A compression check with a leakdown test would be in order before any more parts are ordered.

Also, isn't there a way to verify the base timing? Jump out the ECU and set the base, then give the system back to the ECU? A retarded base can cause running havoc on a little motor.

 

Plugs looked fine last time i pulled them. Maybe I'll pull another one and take a pic. I have done a compression check and gotten 165 +/- 5 psi on all cylinders. I didn't do a leak down check because the compression check showed up good.

I'll have to look into the timing stuff. not sure how to do that. I do know that the problem occurred before I adjusted my valves, and after I adjusted my valves I zeroed everything in and set the distributor. timing seeeeems right. Like I said it idles strong and steady and actually pulls pretty well, just bucks between 2-3000 RPM one the engine warms up.

 

maybe your timing is off if you moved it...it shouldnt be at TDC at idle. I dont know what your timing is set at.

 

The timing is completely controlled by the computer on these Tacomas. It's not like older distributors that you have to turn and set with a timing light. The sticker under the hood would list timing as an adjustment if it were the case. There are still timing marks on the pulley and timing chain cover to manually verify timing with a light.

The distributor coil resistance can be tested with a multimeter, but a good reading doesn't always mean it's working correctly. Other electronic parts in the distributor can fail and not show up with a simple resistance check. Check the coil for any cracks in the packing material surrounding the internal wires. Check for any signs of arcing which can draw the voltage away.

 

I really appreciate all the advice everyone is giving. I don't want anyone to think I am ignoring them, or there advice. I have looked at every post and done tons of research through the FSM based on your guys' advice. I just think I am going to focus on fuel for the moment. Check all my pressures at idle, while driving, after turning off pump. I'm also going to try to send my scope through the plug holes and see if the fuel injectors are leaking.

Like I mentioned before, in the morning the truck starts up instantly with no problems. If I try to start the truck up when it's hot, like 5-10 minutes after shutting it down, it takes like 6-10 cranks to start, and when it does start it almost dies (like fuel still isn't quite there) but then comes up to idle after a second or two. This clue has me thinking fuel pressure or leaky/clogged injectors or something.

btw, got the new TPS on there and adjusted it per the FSM. problem is still there ,but at least it can be crossed off the list.

 

Well boys I think the mystery is solved and I have to give mad props to @DrZ and everyone else who led me to believe this was a distributor/coil problem. You guys are the best! I went ahead and scored a junkyard denso distributor for 40 bucks from an old 95 tacoma. It passed all the resistance checks and the coil looked good. I swapped it in and drove around for a while and the issue seems to be totally resolved!

I should add that the second I took the old one out it was obvious that it had a problem. The coil was a mess. I'm not sure why I never looked at it more in depth, but I chalk it up to lack of knowledge at the time. I have learned a lot throughout this process on 3RZ fuel delivery and ignition systems. A picture of the super cracked coil that I replaced is below....what a mess (I'm also not sure if it's oem, may have been replaced along the way, I see no Denso or TOYOTA markings like I do on the one I replaced it with). And for reference when I do an ohm check on this bad coil it jumps all over the place and way out of spec.

 

Thanks, man. Glad I could help!

The Toyota coils I've seen have an orange/yellow translucent appearance with the Toyota part number printed on them.

There's something about this generation of Toyota distributors, the ones with computer controlled timing advance, that make them cause weird symptoms when they go bad. Maybe the coil starts arcing at certain RPMs and screws up the electronics controlling the timing or something. I'm not sure. The good thing is they don't fail and leave you stranded. The bad thing is they cause symptoms that lead you to believe it's a problem somewhere else.

 

Exactly, even when the MAF goes out or is really acting up you can always unplug it and run in limp mode. That will get you home at least with out the car dying in stop and go traffic. Still a PITA.

 

Yep, that's what the one I replaced it with looked like. It was a yellowish color with TOYOTA written across it. On the top it had the Denso part number. Very happy to have this problem solved, now I can get back to the fun stuff.

 

I think you are right about the timing advance being the main culprit vs actually creating a spark. That would explain why the truck drove fine in Limp mode. In limp mode the ECU doesn't use any sensors (like the cam sensor in the dizzy) to determine timing advance, instead it just flatlines it at 5 degrees. When the timing was stuck at 5 degrees in limp mode or fail safe mode the truck drove smooth. Just trying to tie things together for future people reading this post with similar isues....

 

SOMDTACO I want to thank you for your post. This helped out with my 2.4 96 Tacoma. I was running into this problem and it everything that you did. I was ready to give up on this truck and drive it kill the engine blew up. I the ignition distributor eBay for $54. To see and to hope that he would work. It took me a few tries to get my top dead center correct. The truck now drives like a dream. I bought this car originally from an auction and it had the symptoms. Like I said I did everything that you did but change that one part. So I can never truly experience the Tacoma. Now I'm keeping the truck clean and doing all the maintenance to it. It is a very fun truck. Thank you again for your post

 

I'm glad it helped you out. I know what a relief it is to have the issue fixed. And like you said, now you can just have fun with it.

 

Hi everyone. I just became a member of Tacoma World yesterday and this is my first post. I just got myself a new to me 96 2.7 3rz Tacoma 2 weeks ago. Definitely a fun ride. I can only imagine how much more fun it will be come summer in my new state of Alaska. The information I have gathered off of TW has been very valuable already and it has only been 2 weeks. THANK YOU

Glad I found this post because,

SOMDTACO has hit the nail on the head. My truck has the exact same symptoms. Great idle, good power on the road (good enough for me to feel confident replacing the 285/75/16 slicks the previous owner had on it with some new ones), it just has a hesitation that develops at 2300 rpm's in all of the gears. The shorter gears are easy to power through and get past the hesitation (it persists from 2300 - about 2600 rpm's) but the taller gears become increasingly hard for me to get past the aforementioned hesitation. I found that it will even do this down hills where load is near or below zero.

I was told about the hesitation before I purchased the Tacoma and I thought that I could readily diagnose the problem and get on with it.
It had the original cast exhaust manifold. It was cracked, per normal, and leaking exhaust so I replaced it with a long tube Doug Thorley header. I bathed the throttle body and the MAF or MAP sensor, not sure which one it is, until they shined. I put on a clean Spectre air filter to replace the dirty one sold with the truck. New plugs and wires. Cleaned up the corroded battery terminals. Most recently, I ran a bottle of Sea Foam Spray Cleaner and Lube though the top side of the throttle body.

I am a carpenter not a mechanic. I do understand the basic dynamics of the internal combustion engine.
I am on course to follow SOMDTACO's path of replacing parts as I try to eliminate the hesitation.
I have not replaced anything else (O2 sensors, Cat Converter, etc,...)
I do think that I will try the distributor next thanks to this thread.

My questions now that I have all that out there are,
1. Do I want an OEM distributor? The auto parts chain store in my area sells Spectra and Cardone distributor models new.
2. Do I need the cap, the rotor or the entire thing? Should I start with one, which one, or all?
3. I know that my ignition can be advanced or retarded if I install the distributor wrong. Is there a good thread on TW or Youtube that explains (in detail) the intricacies involved in swapping out the part(s)?
4. What problems will my truck have if I do install it wrong?