Strange Running Issue

[cjsupra90]

Ok guys here's the interesting strange running situation. Using very slow gradual throttle opening, the engine rev's all the way to the rev limiter with no problem. Anything faster or more rapid throttle opening and it starts to rev to about 3500rpm, then falls back to about 3000rpm at which point it sustains but its very broken up (for lack of better term). For anyone that is familiar with 2step rev limiter used in drag racing. It almost sound or seems like the first stage of the 2step is kicking in just to give you an example of what it sounds or seems like. The car is totally undrivable cause it starts to brake up as soon as you put any load on the engine at all. The engine does idle like there is absolutely nothing wrong and like I said before, with real light and gradual throttle opening, it will rev fine all day long and sound great but this is only true if there is no load on the engine (i.e. free revving)

The car is a stock 92 SC 400. I've check every bit of wiring for shorts or open circuits that I could think of that would be needed to at least rev out under load like MAF, TPS, and so on. We have also swaped out the ECU, MAF and TPS for known good units, plus tried the ones on hand in another car which they all worked fine. I've pulled codes and only come up with a 27 and a 29. They are O2 codes and the car is missing the secondary (post cat) O2's, but Im pretty sure that this has nothing to do with it cause the codes have been there for some time and the car has been running for several months without the seconday O2's. The car also had a tune up about a month and a half ago consisiting of Cap, Rotor, Plugs and wires, fuel filter, air filter and well oil change too but thats besides the point and I know all this stuff was done cause I did it for the guy myself.

In the end, it boils down to the fact that it ran fine one day and the next day the problem was there.

I've also checked ignition output which seems to be fine (i.e. doesn't seem to drop spark when it starts breaking up or anything that would lead me to believe that it was an ignition problem). I also checked the injector pulse width which seemed to be fine but did have a stange blip in it as I tried to rev the engine. What I meen is that when you opened the throttle fairly quickly the engine would start to rise and the injection pulse width would start to increase from about 3.1ms to about 4ms, then all of a sudden the engine would start to drop and the same for the pulse width. When it does this, the pulse width drops to about 2.8ms and then the engine starts to break up and at the same time the pulse width jumps up to about 19ms. This would lead me to believe that it was a ECU problem, but like I stated before, we've tried a different ECU and the problem was still there plus we've tried this ECU in another car and it ran fine.

Anyone have any ideas or things to check that I might be missing or forgetting about????

 

[gloverman]

Have you checked fuel flow and pressure? Also scope the cam and crank sensors. Pay attention to the pikes of the voltage and consistance of the signal. Good luck - cheers

 

[cribbj]

Chris, if you blip and release the throttle (closing the butterfly) and then it continues to go to 3500 RPM, then something else besides the butterfly has to be passing enough air for the motor to rev.

I'd be looking at the IAC, or searching for vacuum leaks.

 

[cjsupra90]

Have you checked fuel flow and pressure? Also scope the cam and crank sensors. Pay attention to the pikes of the voltage and consistance of the signal. Good luck - cheers

Flow and pressure seem to be fine but I didn't get a chance to put a pressure gauge on it. I'll have to check out the cam and crank sensors. I didn't really think that they would be an issue cause the ignition seems to be consistant and operating fine........

Chris, if you blip and release the throttle (closing the butterfly) and then it continues to go to 3500 RPM, then something else besides the butterfly has to be passing enough air for the motor to rev.

I'd be looking at the IAC, or searching for vacuum leaks.

John, I think that you might be misunderstanding something in what I wrote. If I just blip the throttle, it seems to react fine. If i open the throttle rapidly to WOT and hold it there from an idle, it revs to about 3500 the falls back to about 3000 on its own, almost like it was hitting a fuel cut so to speak at which point it start breaking up and acting like the 2step thing I described. Another thing to note is the intake get really loud but I think that that just having something to do with some form of reversion or velocity lose happening in the intake cause to the throttle being at WOT and the VE is well below WOT flow rates.... It honestly seems like there is some form of fuel cut or rev limiter kicking in...

Oh something else that seems strange is that the car smells like its running really rich and the plugs are a bit darker then what they probably should be again insinuating a rich running condition. When i monitor the primary O2's the voltage seems to be fine until it I get it to break up. Once the breaking up starts, the voltage drops to between .16v to .19v which would be extremely lean yet the exhaust smells rich. This tells me that its a constant misfire during the break up stage thus leaving a large amount of unburnt oxygen in the exhaust and hence the lean indication from the O2's

 

[cribbj]

Sorry Chris, yeah I misunderstood what you'd said in the earlier thread and thought it was an air issue.

If it's breaking up with any kind of load, then I'd be looking at the ignition system, particularly since you're getting indications that it's running rich, and that should rule out the fuel delivery side.

I know you said you checked the ignition output and it was fine, but did you hang a timing light on every cylinder to check for misfires, or did you put it on a 'scope, etc.? I'd be all over the new caps, rotors and wires looking for something that got pinched, cracked or broken during the tuneup.

 

[livin9it2up1]

My Friend and i had the exact problem on his 240sx with an RB20, Idk what told me to, but i went under the hood and unpluged the map sensor and what do you know, it worked. It reved all the way to 10k rpm (no rev limiter yet). idk if 1uz have map sensors, i actually just got into toyotas so im learning too.

 

[cjsupra90]

Ok, update and problem solved.. First off, good call gloverman on the fuel pressure and flow. Turns out it was the pump. I got a chance today to put a pressure gauge on it and sure enough, with the engine idling it had 30psi which I though was a bit low. As I opened the throttle slowely, the pressure actually dropped down to about 25psi.. If I opened it rapidly, it would drop to just about 0psi, engine would fall out, pressure would come back up to about 20psi, and then fall to about 0psi again. It would constantly bounce between 20 and 0. This explains the simulation of a fuel cut / 2step type reaction I was describing...... I swapped out the pump with an extra walbro that I had and it ran perfect....

 

[gloverman]

Thanks for the reply. We are changing lots of fuel pumps at the moment on all kinds of cars. One of my staff surprised me the other day when after a test drive he suggested a fuel pump. I thought it felt more like an airflow meter but he was right. We have a really good machine which checks flow and pressure at the same time. Good to hear your car is fixed. Cheers

 

[cjsupra90]

Yeah, my first guess was AFM also with the way it was acting, but sure enough it was the pump...... It was really acting like it was hitting a fuel cut (like the AFM was frequency was going way to high and ECU was saying, "no no, this isn't right!!!" I knew this wasn't the case though cause the injectors weren't cutting out when I watched the pulse width that the ECU was triggering at. I guess you could say that it was hitting a fuel cut, just not ECU derived.........