Engine Hesitation above 3500 rpm - any ideas?

[stevetwood]

I have an 89 FC3S n/a (13B EGI) and had a aftermarket exhaust & manifold fitted at RoTechniks (good honest guys who\'ve been servicing it for me for the past two years, IMHO) last week. the engine has done 91K and is reasonable at starting when warm.

I asked them to remove all the exhaust gas recycling stuff catalytic converter, 3rd exhaust pipe and everything not required - air pump has gone too (now with blanking plates here and there).

I picked it up last night and immediately noticed a BIG hesitation from the engine at any point above 3500 rpm when I tried to accelerate moderately hard. This was both when the engine was cold and when warm, made no difference. If I lifted my foot off the throttle and re-applied it gently, it would spin up to the rev limiter buzzer. Progress down the road was slow. My son thinks the new exhaust note is fabulous, I\'m still getting used to it.

I cleaned the plugs up, checked the plug electrics to no avail. RoTechniks have asked for it back at the weekend and as it is my 2nd car, there\'s no problem there.

I\'ve searched the forum (engine hesitation) and noticed an article about the ECU and primary and secondary injectors opening and there being an overlap which causes lean a fuel distribution to the chambers and awaiting some sort of reprogramming of the ECU.

Any ideas from the forum as to
1. Things to try to help diagnosis or give RoTechniks a better idea
2. Things to try to fix it myself - I don\'t want to wait until Saturday if it is simple

Thanks in advance

Steve

[grinder]

ah yes the famous 3500-3800 hesitation !!

its sometimes due to a bad earth somewhere ( a simple fix really )

if you read up on rx7club.com there are THOUSANDS of threads about it

its when the secondaries start to fire or summat like that

taken from a website :

Good grounds are important because poor grounds cause voltage drop. The consequence of this voltage drop is that the systems in that circuit can malfunction. How do bad grounds cause voltage drop? Well, the answer is Ohms Law.

Ohm\'s Law states that V=IR. This means that the voltage drop (V) is proportional to the current through the circuit (I) and the resistance (R) of the resistor (or bad ground in our case). An easy example is attempting to draw 10 amps through a bad ground with a resistance of 1 Ohm. So V=10(1) thus making the voltage drop a whopping 10V. This leaves only 2 volts available (in a standard automotive 12V system) to power the circuit and means that all that extra power must be dissipated as heat at the resistor (bad ground). Generally this heat makes the condition even worse, causing more voltage drop and more heat. In the most severe cases the ground will function under light loads yet fail under high current draws causing what appears to be an intermittent failure. Many people have seen this behavior caused by a loose (high resistance) battery terminal.

In a car with a relatively modern fuel injection like the RX-7, the chief problem associated with poor grounds is that sensor readings can be effected. All the sensors in the RX-7 are ground referenced meaning that the ECU passes a current through them to ground essentially measuring the voltage drop across the sensor and using it to adjust engine parameters. If another resistance if introduced into the circuit this value changes and thus the ECU sees the wrong sensor reading. The ECT (Engine Coolant Temperature) sensor/water thermo sensor, IAT (Air Intake Temperature) sensor, TPS (Throttle Position Sensor), pressure/boost sensor, atmospheric pressure sensor and AFM (Air Flow Meter) are all essentially variable resistors with the ECU on one side and a ground on the other (this is a great simplification but good enough for our purposes). The O2 sensor and knock sensor both generate their own voltages but are still connected to ground on one side with the other wire feeding the ECU through a shielded conductor. These sensor errors can cause all kinds of drivability problems that range from the common 3800 RPM hesitation, bucking on decel, poor performance or even sporadic CEL (Check Engine Light) illumination.

Aside from fuel injection issues, bad grounds can cause poor performance in almost any circuit. Voltage drop can manifest itself as dim headlights, one circuit malfunctioning when another is switched on (a common symptom is that the heater blow slows and dash lights dim when the brake pedal is pressed) and all kinds of other random weird issues.

Stu

[stevetwood]

Thanks Grinder, I will read up and see if I can find a loose earth somewhere

[grinder]

you are best running a big earth lead to the battery and then to the body - just to make sure that its OK

apart from that - you are on your own

Stu

[stevetwood]

Fixed!

Today was the first chance I had of looking at the engine bay in the daylight

One of the secondary fuel injector leads (where the connector is broken and slides on and off the junction easily) had been knocked off.

Hence the engine hesitated under load above 3500 rpm.......:Thumbsup!

I must say, that having lost the Cat, Air pump, exhaust gas recycling system and 3rd pipe, it has made a difference on the performance.

[grinder]

how i love simple fixes

glad you are happy with it now

Stu

[m4drx]

Fixed!

Today was the first chance I had of looking at the engine bay in the daylight

One of the secondary fuel injector leads (where the connector is broken and slides on and off the junction easily) had been knocked off.

Hence the engine hesitated under load above 3500 rpm.......:Thumbsup!

I must say, that having lost the Cat, Air pump, exhaust gas recycling system and 3rd pipe, it has made a difference on the performance.

where was this lead? i\'ve got a similar problem and its driving me mad,

Cheers

[stevetwood]

where was this lead? i\'ve got a similar problem and its driving me mad,

Cheers

Yes it was a loose lead to the secondary injector. Problem fixed

[m4drx]

Yes it was a loose lead to the secondary injector. Problem fixed

can you describe where at all? i might have the same issue but not sure where the lead is or how to get to it.

cheers

[stevetwood]

There are two injectors off of a fuel rail on the drivers side of car. The lead connectors are green in colour and about 15cm apart. You can see them clearly from the top of the engine bay. Try removing the black air intake tube at the manifold and looking under that.

The two electrical connections are one each to the two injectors themselves.

My symptons were hesitation at 3800 revs and above, this is when the secondary injectors kick in, I was later told, and having one rotor not being fed the extra fuel to burn felt like the car was only firing on one rotor when trying to go up through the revs. The lead had been knocked off and needed reseating. Job done. I\'m sorry that I can\'t be more helpful than that.

Check also the other comments in this same thread, especially those from Grinder who suggested to me that one area to try was a poor earth electrical connection.

[m4drx]

cheers mate will check asap