Brice_Brice

So Ive been reading all day about 3800 rpm hesitation(theres a revived post below). Lots of different people say a few different things. I am getting a hesitation in the same range when i put the pedal down and need my secondaries immediately. I need some clarification...

1. I have an 89 turbo and some people say it doesnt happen on later than 88 models. True or False

2. If I can drive past 3800 after a hesitation occurs that must mean my secondaries are engaging and I dont have the same issue?(just a problem in the transition kicking into 2ndaries?)

3. Why do people insist on adding a wire to the pressure sensor when the car worked fine out of the factory? They didnt add a wire there...That means theres an issue somewhere else people are overlooking...

Thanks

RotaryRocket88

iTrader: (7)

1. It's been my understanding that it can happen on S4s and S5s alike. Though it would seem more common on S4s.

2. 3800 RPM hesitation is just that. A hesitation. The secondaries work just fine.

3. Grounding the pressure sensor re-grounds most of the other sensor as well. But this is only a fix for '86 cars. There was a technical service bulletin about it.

Ryan123

iTrader: (3)

Grounds. Your more then likely going to have a bad ground on a 20+ year old car =]

http://www.aaroncake.net/rx-7/grounding.htm

calpatriot

I've owned my 86 NA for over 10 years. It always exhibited the 3800 rpm hesitation. Multiple shops tried and failed to correct it. My car has the TSB-specified additional ground on the boost sensor.

Last week I seem to have gotten rid of most or all of the hesistation--by accident. I installed a new oxygen sensor, adjusted the idle speed, idle mixture and TPS per the FSM. Then I fine-tuned the TPS to read exactly 1k ohms at idle throttle.

I was able to verify that the car was going momentarily lean at the point that the hesitation occurred (green lamp test, & O2 voltage).

Immediately 95% of the hesitation went away. Since then (200 miles) it has improved further, to the point that without instrumentation I cannot tell just by feel if it is hesitating or not.

My working theories are:
1. ECU brings in the secondaries late, after the duty cycle is reduced on the primaries, causing a temporary lean condition.
2. The initial flow rate of the pri + sec condition is insufficient, and it takes the ECU 1/2 sec to adjust the fuel delivery to correct the lean condition
3. A lean condition might be created when the ECU stops the split and port air, and begins duming the output of the air pump into the intake pipe. That is after the mass flow meter, and perhaps the additional air causes a lean condition, once again just until the ECU catches up and adjusts the fuel injection duty cycle to compensate.

I am pondering some test/monitoring set-ups that might be able to determine the true nature of the cause.

calpatriot

Whatever the cause, it seems to be made worse or better depending on the state of tune, particularly the TPS.

Another theory is that the seondary injectors get clogged sooner than the primaries, due to less use. When the secondaries come in, their flow rate is then less than the ECU expects, and the temporary lean condition is created until the ECU can correct it based on the O2 sensor input.

calpatriot

Upon further reflection, I realize that there are fatal flaws with theories 2 and 3. Under acceleration, the emissions system runs (supposedly) in open loop. If that be true, the ECU cannot possibly be correcting the lean condition.

This leaves theory 1, or some version of it.

The secondary injectors are futher from the intake ports than the primaries, so one possibility is that when the secondaries come in there is a finite delay time required for the fuel they inject to make its way to the intake ports. I do not know what the air velocity is inside the intake, but it seems to me that it would be high enough that any such delay would be insufficient to cause the duration of the hesitation that I have seen.

I am wondering if maladjustment of the TPS can cause the ECU to get the fuel injection transition out of phase--that is, the secondaries are not brought in on their 40% duty cycle until some time after the primaries are reduced from 80% to 40%?

RotaryRocket88

iTrader: (7)

The common explanation is poor grounds. But there are many, including myself, that have attempted to remedy the situation by cleaning/replacing contacts, only to see no improvement. My feeling is that it's related to a number of things. I used to have the hesitation under moderate throttle when my car was NA. But once I switched over to a turbo ECU, MAP, AFM, TPS, and clean injectors, it disappeared. All of the wiring remained in place, but the ECU and all of its sensors were replaced.

Going to what you mentioned about the TPS interfering with the injector transition, the TPS WOT value occurs far before WOT actually happens. It'll be at maybe 1/3 throttle. And it takes a certain amount of load before the ECU will switch the secondaries on at 3800 RPM. If under very light throttle, they could come on significantly higher in the RPM range; 3800 RPM is just the minimum value they could be switched on at. So it would seem that the condition in question (moderate throttle) should be where the TPS is already maxed out. I suppose if it was badly out of spec, you could end up with some improper fuel corrections, but I don't think the TPS is used in determining the transition point. From what I've gathered from the pocketlogger guys (Rtek ECUs), there are a number of conditions that must be met before the secondaries will come on. Airflow (AFM), manifold pressure (MAP) & injector D/C should be the most important factors besides the present RPM minimum.

Anyway, if you want to see if the TPS is having an effect on the transition, you can always unplug it temporarily. Other driving conditions will suffer, but you can go for a quick drive to see if the stumble is still there without it. The engine will run without a TPS.

Edit: Also, the injector transition is almost instantaneous. On my Rtek data logs, the switch happens in a fraction of a second. The data points in the logs are around 0.1 seconds apart.

calpatriot

Since my last post I decided to go to lunch, and hooked up my little green light tester to see if the lean spot still existed. I was able to see a momentary dropout (light went off) in the 3500-4000 rpm range, but only for the first two attempts, and then only for a fraction of a second. I could not feel any physical hesitation or loss of power.

During the remainder of the drive, I wound it through full thottle from 3000 -4500 at least 20 times. The light stayed on steady each time; there was no hesistation.

This is in a car that on some occaisions I used to be afraid to turn in front of oncoming traffic because of the hesitation would strand me there for a second or two.

This is not to say that my cure will work for everyone; it is not at all clear that there is but a single cause for the 3800 rpm hesistation.

My process was O2 sensor (& wiring of it); idle speed, & idle mixture, and TPS checked for dropouts (none) and then adjusted for exactly 1k ohms. I did notice that the screw took a number of turns to get from 1.2k ohms (which was the reading after the two-light method of adjusting the TPS) to 1.0k ohms.

I did not work on any grounds.