1999 4runner engine stumbling

[Torkable]

I have a 1999 RWD 4runner, 3.4 liter, and the engine is stumbling very badly when given any throttle. It idles very smoothly, but as soon as the throttle is applied, its stumbles and starts surging. I have tried to diagnose and fix the problem myself by have had no luck.

I started with the cheap and easy stuff:
I changed out all spark plugs, some were too loose, worn, or had carbon blow by on to them.
I inspected, cleaned, and tested the ohms and volts on the MAF.
I changed the fuel filter.
I tested and replaced the fuel pump.
Checked for vacuum leaks on the engine block and all the hoses.
I replaced the O2 sensor.

None of these things have had any effect on the way the truck runs at all so tomorrow I'm going to buy a fuel pressure test kit. The only other thing I can think of is that the fuel regulator is not getting vacuum.

If anyone has any ideas what could be causing the problem I'd love to hear them.

 

[kball]

Do you have a CEL?

I'm thinking possibly TPS...

You changed the plugs.. what about the wires?

 

[Torkable]

Forgot some things.

Yes, the CEL in on, checked the codes, its reading lean on bank 1(only bank I believe), which is why I cleaned the MAF and changed the O2 sensor.

I have not checked the spark plug wires, but should I guess, I believe that it would not idle smoothly if the wires were bad but that could be totally wrong.

As far as TPS goes, I unplugged it and started the truck and nothing changed so I temporarily wrote it off but perhaps I should test it with the voltmeter.

Thanks for the reply.

 

[LittleCaesar]

Ditto on the CEL.

Could be a plugged kitty, or a quite bad head gasket leak.

If you pull the vacuum hose off the regulator, and smell and/or see gas leak out, then there is a problem. Otherwise, more than likely it is fine.

 

[541joseph]

How did these symptoms stArt? Was it over night or did it slowly get worse?

 

[Torkable]

If it was a bad head gasket leak I would have found it, and would most likely be able to hear it. I sprayed all the hoses and all around the gasket to check for a leak.

The symptoms started and got increasingly worse over about three days. One morning I got in, starting driving to work, and it wouldn't go any faster than 30mph, if I tried to give it more gas, the rpms would drop significantly. I started to turn around and go home but the problem suddenly stopped, suspiciously at the same time the engine started to heat up. The next day it was worse and the day after that it was un-driveable.

Currently if I give it any gas at all the rpms drop below idle and it surges and dives real bad. Sounds like its right on the brink of dieing every time.

 

[DeathCougar]

Absolute first thing I would do is a compression test. Make sure you don't have an internal engine issue. Takes a few mins, plus it's a good baseline to get an idea how your engine is running. If that checks out, move to the next thing.

Lean bank One tells me bad Fuel injector. Very common on our engines. Ohm the injectors out, make sure they are working properly. Also possible you have a blockage in the injector.

Check the coils, and make sure you are getting spark in every cylinder as well. If you didn't throw a P0300 code, the coils should be OK but it is always a good thing to check them.

Throwing parts like fuel pumps and O2 sensors at a problem like this is merely costing you money and getting you nowhere, as you have discovered. Inspection of the spark plugs you pulled out the first time should have shown one was different, based on how the engine is running. I also highly doubt its a plugged cat. The style of cats in our trucks don't clog very often (never seen one ever) as the honeycomb just breaks down.

 

[Torkable]

Just got done with a few tests.

Performed compression tests on each cylinder with a Harbor Freight compression tester. On a previous vehicle the pressure came in low and consistent also. seems like the tester likes to read low but the pressure was consistent ranging from from 117-123 psi.

When I took the new plugs out, there was a decent amount of what looked like oil on a few of the plug's threads. As I mentioned, the plugs I replaced originally were very loose and had carbon on some of the boots.

Tested some of the electrical. The coils(both primary and secondary connections) and wires are all showing the correct ohms and getting 12 volts. I have not tested the injectors though.

Going to pick up a fuel pressure test kit, I found one that has an in-line connection and its only about $20.

 

[LittleCaesar]

How many miles are on this engine?

What numerical value code was produced thanks to the CEL (ex. PO300)?

 

[mikebontoft]

if you had oil on the threads, it was probably misfiring at least in part by leaking valve cover gaskets. otherwise...




MEOW!

 

[DeathCougar]

You replaced the Fuel Pump, and the Fuel Filter...and still think its a fuel pressure problem? hwell:

What is the deal with the oil, that was never mentioned before. That can cause a misfire without throwing a code.

 

[Torkable]

Ya I didn't notice oil on the plugs that I replaced, possible because they were in bad condition and had carbon blowing past them and were filthy all over. Today I took the new ones out to do the compression test and noticed that 2 were squeaky clean and the other 4 had oil on the threads.

I guess I'll forget about the fuel pressure for now.

Also, its at 136K miles.

 

[DeathCougar]

Ya I didn't notice oil on the plugs that I replaced, possible because they were in bad condition and had carbon blowing past them and were filthy all over. Today I took the new ones out to do the compression test and noticed that 2 were squeaky clean and the other 4 had oil on the threads.

I guess I'll forget about the fuel pressure for now.

Also, its at 136K miles.

A little bit of oil isn't really a big deal. But COATED in oil, or a lot of oil in the cylinder is a very big deal. If you see a pool or puddle of oil in the tubes, you have a problem.

 

[mikebontoft]

yeah oil will fill the spark plug wells and the spark will jump through the oil to ground instead of the plug. the fix is valve cover gaskets.