I hate to start yet another thread about hesitation & stumbling but I really need some help on this.

Here is the breakdown:

My 94 with the 3.0 has 160K on it. At about 140K i noticed that on a rather hot day I had some hesitation accelerating out of 2nd gear. It progressively got worse over about an 8month window(more pronounced and noticeable) From basically a stop to just under 2K RPM's it bucks and stumbles on anything but the lightest of take offs. Feels like its horribly out of time. Then at about 2K RPM it takes off like a rocket, and runs just fine. I have had the rig since 40,000 Miles. So I decided it was past time for a timing belt replacement, I did that work. Made no difference as soon as she heats up about 10-30 mins of driving same issue. I took it to the dealership and they said that it needed a valve job which I paid them to do. No change. Took it back, they replaced the TPS, MAF, and Fuel regulator - No fix. Shortly after the head work I lost a head gasket. Again I took it to the dealership and begged them to get to the bottom of the acceleration hesitation. Another $1500 and still same issue. I replaced the Igniter, Coil Pack - still no chage. The dealership said the problem is the fact that I use Bosch plug wires, Cap & Rotor. And told me that its a known issue and I have to use OEM parts here. So I forked over another $200, and had them replace the plugs while they were at it. (that they were suppose to do the first time the did the heads) No change. Over the last year 1/2 since then, its been fuel pump, filter etc. I even borrowed the ECU out of my father in laws 89 3.0 and that overall did not work well but... you could still feel the problem on acceleration with the added bonus of a bunch of other scary sounds and noises.

I have had it to several other "Toyota" mechanics in town to no avail! I am just about to trade this thing in on something more domestic, but I don't want a payment! I am convinced that it is something simple and stupid!

Here are some other facts i have narrowed down.

No trouble codes, none! nada.

Once it is really warm, the Cruise Control will not lock in. it just blinks the light. Sometimes it will work.

At about every 3,000 Miles I have significant burning and char corrosion in the distributor cap and rotor.

I periodicity see the brake light flash when I do hard accelerate, but I cannot duplicate this as a trend. I had a complete brake job, and still see this.

This is a 5 Speed Manual Transmission. I just did the clutch a couple weekends ago. and for a 160K it looked pretty good.

Burning Premium "Chevron" gas seams to have a small effect on the hesitation. Octain boosters have not shown much effect.

When she is stone cold she runs like the day I brought her home.

The dealership last adjusted the timing, and said I had it all screwed up. I pointed out that they were the last ones to rebuild the heads twice.

I still have noticeable valve chatter when going down the road. Dealership says this is normal.

Dealer also claims that they replaced the knock sensor and pigtail when they did the heads.

I live in dry/hot Central Washington and I use HEAT in the tank every 4th or 5th fill up to keep the ethanol issue in check. I started this practice after the problem started. It has no effect.

I have read every single forum and post and not a single thing has unearthed an answer. I am hoping that someone can help me!

Your reference to the cruise control not operating makes me wonder about vacuum leaks.

If there is a leak it might lean out the mixture enough to make it not run right too.

Randy

I have checked every hose, and there are many. There dose not seam to be any vac lines to the cruise servos. And again its only a problem after about 30 mins of driving. Anybody know what the exact logic changes occur between a cold engine and a hot engine? I assume that there are things that change for timing, and mixture after a certain point. If I knew the exact process I might have a better understanding as to where to look. I have theorized that it could be a vacuum leak on something that only opens at temperature? The guy at the dealership chuckeled at this, and said "a vacuum leak is a vacuum leak - if it were a hose you would have the problem all the time or very intermittently". But knowing all the black spaghetti on top of a 3VZE, I am sure some of that stuff makes changes after reaching a certain temp.

Here is a side question... is it possible that there is a problem inside the brake booster diafram? how would I go about testing that? I have a really fancy vacuum gauge but not sure how to test something like that.

I had this issue a few years ago on my 22RE. Turned out to be a hairline crack that you could barely see on the intake tube rubber boot around the throttle body. Have you checked both boots on this tube......thuroughly?

I have inspected those many times, but its probably worth a new look. I would expect a leak there to be constant, but at this point ill try anything.

I just spend another hour under the hood. I checked the brake booster and check valve, and a bunch of other hoses. I did find a small hose i never noticed before with a bad end on it for sure. I but neither the hose or it's port had enough vacuum for me to feel it so I am not sure what that was about. I fixed it, still no change.

One thing i did notice is that fuel rail pulsates, like a fast heart beat... anyone confirm this is normal?

Check my math here please.

I am running under the belief that if this were a fuel delivery problem that I would see this hesitation and stumbling at higher RPM and or under load, and it would be worse the higher I took the RPM's. Since there is an exact point between idle and 2,000 RPM that I experiences this phenomena and that it jets off like a rocket at that exact point in the band, that this would not be a delivery issue.

Anyone have experience saying this is not necessarily the case?

No problems with the cold intake hoses. I even went as far as sealing up all the fittings with some RTV. I misted carb cleaner on all the hoses and connections, and did not detect any leaks.

Not the brake booster or or the hoses to that. Tested all

Vacuum test looks good on the intake manifold, nice and stable.

Dumped a can of SeaFoam in with a fresh tank of gas 50 miles and no change.

I have a bit of a problem with the word 'claims'. Don't happen to have the invoice to check for the parts?

Also with it being heat-related, am wondering what a leakdown test would tell you. How long ago was the last head rework done?

Might try hooking up a fuel pressure gauge and/or an air/fuel ratio gauge and see if you can correlate the hesitation with a change in the above readings:
- http://www.4crawler.com/4x4/CheapTricks/Gauges/index.shtml

If those are both OK, then it may be the ignition since you report problems there.