Help! Untraceable Ignition Problem

My truck is a 1995 4 cyl 4x4 single cab. It only has around 192k miles on it but has now been in the shop for about 2 months. It has a misfire under load that has perplexed the mechanics and myself. It hasn't been running quite right for the past year and a half, and it developed this a few months ago. The mechanics say the ignition system will just cut out randomly above a certain RPM. It starts and idles smooth.
Here's what has been replaced over the past two years:

• Spark plugs
• MAF sensor (Delphi)
• Fuel filter
• Cap and rotor
• Spark plug wires
• Battery and battery terminals
• Catalytic converter, oxygen sensors, and muffler
• All vacuum lines in the engine bay
• Ignition coil (NAPA Proformer)

I also put 3 different fuel system cleaners through it.
Here's what the mechanics have replaced:

• Cap and rotor (apparently mine were poor quality)
• Harmonic damper
• Condenser
• Ignition coil (NGK)
• ECU (refurbished)

Timing is perfect, and the igniter and relevant wiring are in good condition. They said the pickup rotor is fine as well.

Any help I can get would be great.

 

How is the fuel pressure?

 

Fuel pressure is fine

 

This may be a longshot. The 3RZ engines are known for the exhaust manifold to crack that can cause it to run rough under load. Might want to check around for any cracks. Also, have you done a compression check? Might also want to Google around for 3RZ valve or ring problems. Good luck and keep us updated.

 

My mechanic has said it’s an ignition issue but I’ll mention the manifold and compression testing. Do you know if either of those could result in a misfire reading when looking at an ignition scope? He says the entire ignition system (i.e. not cylinder specific) just cuts out on the scope when it happens

 

So they have a scope it sounds like, have they checked all the PCM inputs like cam and crank signal, powers & grounds, and outputs like ignition control and injector control?

 

They replaced the ECU itself and said that wiring between the ECU and ignition system was good, but as for individual inputs and outputs I don't have a list. They said when they tested the old ECU that two of the cylinders were getting too much fuel and two were getting too little, but when they switched out the ECU, the misfire remained. I plan to ask them about the crank sensor and pickup rotor as they're the only two components in the ignition system they haven't inspected. He's inspecting the distributor again. As for power and ground, I'm not sure, but he said they cleaned up bad connections at the ECU harness with little effect.

 

So did the mechanic ever verify the compression? And have the valve shim clearances been checked?

I had a similar experience with intermittent misfire. One was low compression in one cylinder which improved with a valve shim adjustment. The other misfiring cylinder is very seldom under load but has good compression with a bit of oil in the cylinder. Haven't fully diagnosed that one yet but I suspect it's likely the oil preventing spark.

 

The engine was compression tested and was good in that regard. I told the mechanic about the exhaust manifold issue STOCKTRD mentioned and he said he’d look into it

 

No mention the injectors have ever been checked was that forgotten ?

You don`t mention what Codes are being tripped. Ignition would be PO300 codes.

 

I think they checked the injectors, but it would have been a while ago so I don’t remember for certain. I doubt they didn’t. The misfire is erratic and not specific to a cylinder, so injectors seem like a dubious answer. My system is Toyota OBD-I not OBD-II, and these symptoms presented themselves without a check engine light.

 

So you have a pick up not a Tacoma with the 22RE I think we all missed that.

Completely different your 3rz in your name confused me then so many will call a 95 a Tacoma

I would look real close at the wiring harness since your problem is intermittent I still say with no light it is more fuel related then ignition . The normal blinking signal was seen when the codes were checked??

Between pick ups and Celica`s I have a bunch of these engines.

 

No my truck is a Tacoma. The engine is a 3RZ-FE. The 1995.5 Tacomas are not OBD-II compliant. They use Toyota OBD-I, and are the only year to do so. I believe the code they found was code 14 (ignition signal).

 

So they have an ignition code and it's intermittently losing ignition, why aren't we chasing that instead of everything else?
If the shop has a scope and knows how to use it and test circuits I wouldn't think it would be too hard of an issue to track down.
Has the igniter (ignition module) been tested/replaced?

 

Anymore far to many shops just guess and replace parts then do any trouble shooting .

 

You'd think it would be simple but it's given them an astonishing amount of grief - they said they've never seen such a spectre of an issue with a Tacoma. They've been running through the entire ignition system. They checked and/or replaced everything typical which is why they tried the ECU. They said the igniter signal was fine but I'll ask them again. I'll also ask them to check power and ground signals for everything, though they said wiring seemed good.

 

Just got word from my mechanic, things have changed. They've ruled out the ignition system entirely and are set that the issue lies in the fuel system. He mentioned that before the ECU was replaced, two of the cylinders were getting too much fuel and two weren't getting enough. After swapping it out, the condition improved but that issue still remains. An excessively rich mixture in some cylinders would explain the bad ignition readings, as the spark would fail to fire through the mixture. This completely changes the situation.

 

Have they taken a fuel sample and checked for water contamination?
Are you running E85 ?

You state the truck idles fine only misfires under load? What kind of load? Normal acceleration or pedal to the metal?

 

I'm running regular 87 octane gasoline, and the misfire occurs under normal acceleration.

 

If the shop has the ability to check the fuel for alcohol content have them do it. Also have them pour some fuel ( about a cup full) into a clear container and let it sit for a few hours. If its contaminated with water the two liquids will separate. You will see two distinct lines in the container, one where the fuel is and one where the water is.

Is your truck a manual or automatic?