Chrisz

==Updated==

Ok, so I've read all the threads I could find on this and I still am unable to fix it. I have a lot of experience with these motors, but this has me stumped.

Before I started all of this, the truck ran through the whole rpm range, but lacked power.
So I checked timing and I couldn't get my timing set right, couldn't rotate the distributor enough counter clockwise and the jumper wire did not change my timing at all. So I removed the throttle body and adjusted the tps according to the fsm specs. Put the throttle body back on and now the jumper wire worked as it should and I could set the timing correctly. In solving this problem I think I've created another. Now when I start my truck up cold, it will idle fine but if I rev it up it cuts out at 3000 until it drops a few hundred rpms. When the truck has warmed up it does the same thing, only now it won't rev over 1500 rpms. Checked and rechecked the tps, replaced it with a new one, checked the tps signal at the ecu, no problems there. While I was doing this work it was drizzling rain outside, so I figured maybe I got some sensor connection wet. So I detached vafm, tps, distributor connectors and dried them out with a heat gun. No change. Pulled dist cap, bone dry underneath. When I unplug the tps the rpms change a little so i suppose it is doing its job.

I don't know what to check from here. Any opinions? Thanks guys

JasonYota

iTrader: (1)

If your saying you couldn't turn the distributor enough to get it timed right. That sounds like its off a tooth

Chrisz

I should clarify. Timing is dead on now. The tps was misadjusted by the previous owner and it did not allow me to set the timing correctly. So right now timing exactly where it should be and the distributor adjustment bolt is near the middle of the slot like normal.

RJR

There is a rev limiter on these engines, that should cut in around 6500 rpm, and it's interesting that your rpm limit seems to be about 1/2 or 1/4 of that, depending on temperature. I suppose it's possible that the ECU is getting double/quadruple pulses from the distributor and thinking that the rpm's are much higher than they actually are. I have no immediate ideas as to how to check this without an oscilloscope, but it's a possibility. A bad ground could possibly cause noisy pulses to the ECU that it could interpret as having multiple edges.

Does the rpm limiting happen when in neutral, or just while driving with the engine under load?

Chrisz

It happens regardless of being driven under load or in neutral. It seems related to throttel position. The only factors involved were rain, and me removing/adjusting the ththrottle body and tps. It's going to be embarassingly simple I'm afraid...

RJR

When you say it "seems related to throttle position", what do you mean by that? The throttle position should be different at 3000 rpm unloaded vs. loaded.

As you advance the throttle up to and through the problem point, do the rpm's stay fairly constant as you continue to advance the throttle, or does it stumble and nearly die?

Chrisz

Yeah I guess what I said doesn't really make sense because it would be different as you said. It runs perfectly up to when it cuts out. Even seems to have really good power up to that point. They it suddenly just cuts.

jennygirl

Sounds very possible that it's still an issue with TPS. The revving to a certain point is telltale from my own experience.

Did you use a feeler gauge to adjust it, and per 4crawler instructions?

http://4crawler.com/4x4/CheapTricks/TPS/index.shtml

In my opinion, TPS is the hardest adjustment to get right on the car.

Chrisz

Yes I did. Probably 5 or 6 times just because I was so convinced that it was the problem. I even probed the wires at the ecu and monitored the changes with the throttle input to verify that I didn't have a wiring issue.

RJR

If the problem doesn't change when the TPS is unplugged, as the OP has indicated, then I would think it's unlikely the TPS, or the TPS adjustment, is at fault. All the TPS adjustment really affects is the idle close switch, which tells the ECU that the throttle is at idle position and to go into an idle management mode instead of normal run mode, for better idle stability.

ChrisZ, if you have a noid light, put it on the injectors and see what they're doing when the engine starts cutting out. As bizarre as it seems, it still feels to me like the ECU is cutting off fuel for some reason, such as thinking the rpms are exceeding the rev limit. The fact that the rpm maintains the same level whether loaded or unloaded indicates something is actively controlling the engine speed - it's not just a matter of fuel starvation limiting the amount of available power, for example.

If you're into electronic techie things and want to go to the work of getting my diag port reader up and going on your laptop, it will tell you what the ECU thinks is coming in from it's various sensors and may give you a better clue as to what the problem is. https://www.yotatech.com/forums/f116...reader-278801/ It will tell you what the ECU thinks the rpm is, for one.

Chrisz

Ok I'll try the noid lights. Hopefully it doesn't come down to having to use your software, but if it does I will. Crazy thing is it is related to the temperature of the motor. Warmer the motor the lower it revs. Randomly 2 times out of the 20 or so I just revved it up it revved up to 4500. Weird...

jennygirl

Wow RJR that is really cool. I've been wanting something like this except with an arduino. Super duper neat

Chrisz

So somehow it is fixed now...I noticed that my timing was way way advanced. However I knew that it was set correctly because I just set it yesterday. So I started fiddling with the tps and then I unplugged it and plugged it back in. And all of a suddsudden everything works great. I don't know of it was the tps or not. I have unplugged it probably 30 times in the last few days so it would be weird for this time to be different. Perhaps a wet connection somewhere finally dried out. I have no idea. But I works for now so I guess I'll see how it does tomorrow.

Chrisz

Ok just as I feared, the problem is back this morning just as bad. This time messing with the tps has no effect. I dont know what is going on.

RJR

Seems like you have an intermittent connection somewhere. Check for open grounds. Also, there is a shield on the wiring from the distributor to the ECU. Make sure that shield is solidly connected to ground.

RJR

With your SW skills, jennygirl, you should be able to whip the arduino into doing this in fairly short order. It would make a great accessory to permanently mount in the vehicle, or to have as a diag tool for your Toyota friends. All of the info you need to interpret the data stream is in the zip file.

jennygirl

You are too good! I think you may have just convinced me. I'm envisioning a 16x4 little led backlit character panel atop an arduino doing exactly this.

Can't wait to head home to see if my truck has the TE2 port. I've been reading that some do not?

Sorry, don't mean to hijack this thread!

Chrisz

I should mention also that when I unplugged my tps last night the idle went up and then it went down when it was plugged back in. And that's when my timing fell back into place and it ran great. That was the first time that my tps being unplugged had any effect. Up until then and including today, nothing changes when the tps is unplugged. Not sure what that means, but those details may be important to someone.

Chrisz

I decided to go ahead and ohm the vafm per the fsm instructions. Everything checked out except for the reading from e2-vc. Between those two I had zero resistance. So I guess I'm off to look for a new vafm. Could a bad vafm cause this issue?

RJR

Hard to say. Were you testing it with the connector unplugged from the VAFM? I would expect a CEL code if the VAFM is messed up like that, but only the person who programmed the ECU would know for sure.

What you can say is that, if any of the sensors are totally out in the weeds, the ECU really doesn't have a clue as to how to manage the engine, and your vehicle has just turned into a science experiment with no predictable outcome.

It might be instructive to measure the VAFM voltage at the ECU as you move the vane, much as you did the TPS. You should see a smooth voltage from 0 to 5 volts. My guess is you won't, given that measurement.