First off, I have a late model year '88 S4 GXL. Fresh engine rebuild, auto to manual trans conversion, no emissions equipment, new injectors, new fuel filter, new fuel pump/sock filter, clean grounds, fresh plugs and wires.

I have an intermittent 5k RPM hesitation. It goes away if I turn the engine off and start it back up again; but later on it comes back. This only happens when the engine is warm, as I don't get into the secondaries when it's cold. Only parts on the car I deem questionable are the AFM and MAP sensor; both of which were taken from older S4's. AFM was taken from an 87.5 manual car. MAP from a late 86 (sold as 87) also manual car. I'm worried that this MAP isn't compatible with my computer.

Any help would be appreciated.

 

An NA sensor is uniform for all NA S4's. If you happen to have an S4 turbo sensor then that would not work.

 

Well I don't have any turbo parts. So that probably rules out the MAP sensor... it worked correctly in the car I pulled it from. So what would cause this problem I'm having? When the car isn't moving, if I hold the pedal to the floor it jumps to 5000RPM, then bounces between 5000 and 6500RPM (like some sort of weird rev limiter). When I give it a load (say in 3rd gear for example) it pulls hard till 5000RPM, then falls on its face. If I shut the car off and start it back up, the problem goes away and she runs great all the way to 7000+RPM... 30 miles later it comes back, and I've got a 5k rev limit. One weird thing I did notice was when I first installed the engine and went to start it for the first time, the ignition switch or main relay wasn't working correctly. All the idiot lights flashed and shut off, then came back on. I had to cycle the key a few times before it would even start. Could a faulty ignition switch cause this weird RPM problem?

 

Problem is continuing to get worse; to the point where I'm not going to drive this thing till I figure it out. It has to be something wrong with the secondary injectors or something that initiates their function (CAS?). Engine just doesn't want to run past 3500rpm now without lots of bucking/misfiring.

 

Does the car rev freely when in neutral (secondary injectors are not used to do this)?

 

It depends on whether I floor the pedal or ease into it. If I floor it, RPMs spike. If I ease into it, it will peg at 5k and bounce between 5k and 5500.

 

So, w/the car in the driveway and in neutral (not in gear) it behaves this way. If so, then the secondary injectors are not causing this because these injectors only work if the rpm is above 3800 rpm and the car is under load. When free reving the car should not be under load.

Are the injectors stock sizing? All should be 450cc. Is the fuel pump a stock replacement suited for an NA? What happens in neutral if you unplug the TPS? Have you also set the TPS to read 1 volt (Green/Red wire) w/key to on after the car is completely warmed up? Does the pressure sensor's Brown/Red wire read 3.5 to 4 volts w/key to on and plug connected to the sensor?

 

Is there an issue with the 5/6 ports? The hesitation at 3.5k-5k, is it just bogging for power, or will not rev past that at all?

 

Injectors are factory output, correct impedance, less than 2 years old. Same story with fuel pump, factory replacement; brand new. Haven't unplugged TPS. Can't yet, it's currently hardwired with solder and heat shrink (don't ask) soon to have a weatherpac connector... TPS was set using checker lamps and Initial set coupler per FSM instructions. Haven't checked the pressure sensor yet. 6PI works as it should.

 

You might have a poor connection at the TPS (cold solder joint) causing your problem.

 

I guess the best course of action for checking this would be to check TPS resistance at the CPU connector? I'm not ready to install the weatherpac connector yet; need to buy solder and things first.

 

TPS connects to the ECU and the ECU receives a voltage signal not a resistance signal. A cold solder has the ability to work some times but not at others w/respect to passing along the proper voltage value.

You could also tell if the secondary injectors are incorrectly turning on when free reving (which should never happen). Pins 3H-Light/Green/White wire and pin 3F-Light Green/Red wire are the secondary wires at the ECU. W/the engine running in neutral they should each read 12 volts regardless of rpm level. If you rev the engine higher and hold the rpm at or over 3800 rpm or so and the voltage drops to 6 volts then that tells you they are activating when they should not. The TPS, boost sensor and AFM are responsible for detecting load on the engine.

 

With the battery disconnected I can't unplug the connector at the ECU to check TPS resistance? It should read the same as if I were checking at the TPS; if it doesn't that means there is a cold solder joint... Section 4A of FSM gives specs on this resistance at throttle closed and fully open. It also says this:

On vehicle inspection
Operation Sound
Secondary injectors

1. Disconnect vacuum hose from boost sensor

2. Disconnect throttle sensor

3. Start engine. Raise RPM to over 3,500 and check for secondary injector sound with "sound scope"
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Well I guess I've got some work to do on the TPS. I need to be able to disconnect it and/or ensure it's proper connection before I proceed with anything else... no sense in fooling with injectors just yet if the TPS is the first suspect. Not sure what the solder connections look like.

 

What you want to see if the secondaries come online when they aren't supposed to. You should be able to free rev in neutral w/o any problem. Only the primary injectors work when free reving. If you prove the secondaries are not engaging then you possibly have an issue w/the primary injectors.

The wiring of the emission harness is plugged to the TPS and runs to the ECU. You would need to take into account the resistance of this segment of wiring if you were to test the ohm reading after unplugging the ECU plug housing the G/R TPS wire. The ECU wants to see 1 volt so I don't understand what the hang up is.

 

Have you checked the quality of the ground connection on the rear rotor housing?

 

No hangups Satch. I'll be checking TPS later this evening. At work now...

I wish people would read the Original Post before they ask questions. Under hood grounds are clean, this is my 2nd FC. My first one was a corrosion nightmare; I learned my lessons on bad electrical connections.

 

You should also test the voltage sweep of the TPS as it should range from 1 to 4.5 volts or so. And to test the sweep you would at least need an analog meter to perform this test.

 

If you haven't double checked that specific ground you're pissing in the wind, it can cause your symptoms. You don't even have to pull the intake manifold to get a wrench on it to ensure it is tight.

Also, try to be a little less pompous when you're asking others for help. Just because you say you made sure they're clean doesn't indicate that you double checked them or did anything other than a visual inspection. "Clean grounds" can mean anything.

 

I know the ground on the rotor housing is clean/tight, but I'll check it again. Does the ground from Trans to firewall even do anything? Isn't there one by the starter? Both are hooked up but I didn't scrape the paint off under it on the firewall before assembly.

Still haven't checked TPS; too busy with finals, and fixing everybody else's RX-7.

 

Okay... finally got some time today to check on the TPS voltage. I installed the weatherpac connector on the TPS on Saturday, and checked the TPS resistance and sweep while I was at it. The resistance between two of the terminals didn't come out right at WOT. I replaced the TPS for one that more closely fit the FSM specs.

Anyway, back to today: I checked the voltage on the ECU pin 2G and it reads: .4vdc all the time. Key off, key on engine off, key on engine running. All with engine warmed up. Always .4vdc. What the hell.


Edit: must be checking the wrong wire... I checked the one opposite "2G" and it seems to be the correct wire...

 

OK. TPS is set at 1v, at idle engine fully warmed up. Secondaries both read approximately 9v regardless of RPM when free-revving (car is stationary.) as the car runs though, this voltage seems to be falling. As I write this, it's down to 8.7v... car at idle.

Edit: drove the car around for a while. Secondary injector voltage at idle is still steadily falling. They came online while I was driving; I could see the voltage drop at WOT above 3.5k RPM. At idle right now the voltage has dropped to 4.1v.

 

Well... I found out the ECU chassis was not a reliable ground. I was previously using it instead of the pin on the connector. Now I'm getting the proper voltages. D'oh! *facepalm*

 

If you see the voltage at the ECU secondary wires as being rather low then perhaps you should test other pins at the ECU that relates to sensing load such as the TPS, the Boost sensor and the AFM.

 

I checked this voltage incorrectly. When using the ground wire on the harness connector, I found the voltages to be correct. They all fluctuate as they should. TPS is correct. AFM is correct...

So after checking voltages till my face turned blue, I decided to take the car out for a spin. It ran horribly. (voltages still good on everything.) so I decided to connect to the O2 sensor, and see what it was doing. Sure enough, it's leaning out. Under acceleration the O2 sensor drops to zero volts and the engine chokes up... yet the injectors are getting their respective signals. Fuel pump is moving fuel (I can hear it flowing when I jumper it with the car off.) so this is pointing me towards faulty injectors... any other ideas?

 

Well I think I can revive this thread and bring it to a close now... after extensive testing, I decided to check fuel pressure. Keeping in mind the fuel pump and pre-filter are only 2 years old, I was not expecting to have low fuel pressure. After driving the car for about 10 minutes with the pressure gauge reading over 35psi and fluctuating correctly with the FPR, I did a full throttle run and as I climbed over 4k rpm the pressure started to drop. The more I pushed it the more the fuel pressure dropped. At idle it would climb back up to normal. It got down to 5psi before 5500rpm