rough running, high fuel consumption, please advice

[richardix]

I am a newby and I drive my Chimaera (400 Serp HC) for a few months. She drives great but seems to be rough at idle and low in torque. It is slow from idle to about 2500 RPM. Above 2500 RPM the car goes off. The first trip my fuel consumption was about 28 mpg. The last ones it dropped to 20 mpg, without pushing her, just cruising.

What I did so far. Complete electronics have been replaced (plugs, ht leads, distributor, rotor, coil). Checked timing (28 degrees above 3500 RPM). Also replaced the cooling water temperature device.I measured fuel pressure, which is 2,5 bars and also regulates (or drops) if vacuum is applied. All within the right figures.

If I connect it to rover gauge, all measurement I/O's are in line. Lambda trim between odd and even bank differ. Even bank is about -14, odd bank is -89 (according to the log file I made with rover gauge).

My question is if I can suspect an faulty injector, or can anyone suggest something different I have to look for ?

Thanks in advance. regards, Richard

[TV8]

If you can get an infrared thermometer check all the exhausts manifold pipes are similar temperatures. If you still have extenders, particularly new ones, they might not be!

That doesn’t help how they run.

[blitzracing]

Have a look at the long term trim- see if you have high values- this could show a misfire or a bad sensor input, and check the AFM reading in direct at idle make sure its in the 30-35% region depending on engine size.

[richardix]

Thanks for your input.

unfortunately I don't have a (laser) themperature device. I will buy one, to check temperatures. What I did is check all the spark plugs, most of them are white with few black carbon spots. 2 plugs are 99% black. I use standard NGK B7ECS plugs. so maybe these injectors are suspecious.

The AFM is approx 32-33%, so that seems to be ok.

In my Rovergauge log file, I can't see if it is long term of short term trim. I will test that again (if the weather is getting any better this weekend ;-) )

If I disconnect vacuum of the fuel regulator, lambda trim becomes more negative and pulse length of injectors becomes shorter. So, lambda is measuring a fuel increase and adapting to it via ECU. Little effect on Idling, so therefore I assume lambda readings are ok

[blitzracing]

Problem is the ECU has a huge range of adjustment, so you can have an underlying issue and the short term trim will still cycle, so it sort of looks OK. This is why its worth looking at the long term, as the ECU may be making big corrections to cover something else. A misfire will cause very high long term "add fuel " readings.

[WOO5IE]

Try running without the plug extenders.
A notorious problem .

You may need to use resistive plugs if you run permanently without them

[richardix]

Thanks for the advice regarding the long term trim. I did some testing, with interesting results. With the laptop and rovergauge took the TVR for a ride.

- removing plug extenders does not have effect on the engine.
- long term trim is indeed -89% odd and -57% even.
- with the figures of the log file I made some graphs in excel

a. correlation between MAF and trottle position is almost lineair. Seems to be ok
b. correlation between engine speed and idle bypass position, is totally not in line
b1. lambda in time is moving to -255
b2. idle bypass position is decreasing in time. After 30 minutes of driving, idle bypass position is decreased from 0.75 to 0.3. At the same time lambda is increased to -255.

if this is correct, the mixture seems to enrich in time due to closing idle bypass position. And after 30 minutes the injector pulse time is not able to correct it.

Can this just be a stepper motor failure ?

[richardix]

I have included some graphs which I made with excel from rover gauge. The trends seem ok to me. Only lambda trip after 15 minutes. But this seems to have less effect on the pulsewidth.

Also I get an error code 48, Idle control error.

[blitzracing]

The long term trims are high - you say -ve trim values- just check against RoverGauge if this is adding or removing fuel? By default removing trim is adding fuel, so it thinks the mixture is lean, or it can be unburnt mixture- the lambda probes cant tell the difference between un burnt fuel or a lean mixture. Both are high in oxygen, which is what the probe measures. I think you are seeing the ECU doing what its designed to do, but its masking the basic cause, of something like an air leak or general misfire. It wont be a faulty stepper.

Edited by blitzracing on Tuesday 5th December 21:40

[richardix]

Thanks Blitzracing !

The long term lambda is negative, so the ECU should be adding fuel, if I interpretet your post correctly. If the engine idles it looks like a misfire. I can image if it is just a lean mixture and the ECU is adding fuel, it should not having a misfire. Besides 20 MPG fuel consumption seems to me like a lot, so I would assume it gets sufficient fuel (fuel pressure is ok). If I enrich the fuel by taking of the vacuum of the regulator, it does not affect the idling (meanwhile I measured the fuelpressure increasing). Agreeing that the sensors and ECU seem to react normally.

Understanding Blitzracing's conclusion, lean or unburnt fuel, I probably have to look for a mechanical problem, as I completely renewed the ignition, 500 miles ago. I will check earthing, but I am afraid it is something with more impact. Leaking injectors, or maybe camshaft problems (which may cause an badly burnt fuel). I thought I read the 4.0 HC version is more prone to camshaft problems and since the car has ran 50.000 miles.....

So during Christmas holiday I will remove Plenum and measure the heights of the valves/ push rods. Will try to look at the camshaft for wear signs.

Keep you informed, thanks for your advice, much apprectiated

[blitzracing]

Before you rip it all to bits, go find someone with an old style crypton tuner, or a hand held HT tester- there is a wealth of info from measuring the HT voltage down each plug line. An open circuit shows up as high HT voltages, and a short as low. The 8 HT leads are different lengths, so the voltages will vary a bit, but they should be within a few kv of each other- typically around 12-15 k at idle. An open circuit lead can reach 25kv plus, and a short will be a few KV only. You can get simple inline HT testers that plug on top of the plugs for not a lot of money, for just a display to say there is HT reaching each plug. Check Ebay HT testers.

Edited by blitzracing on Wednesday 6th December 20:47

[WOO5IE]

Perhaps worth looking at the engine or when dark at night to see if there is any HT tracking

You can sometimes hear it as well.

[Coefficient]

This issue sounds quite similar to my Griff though I would say that the misfire clears at closer to 3500 rpm. I’d quite like to have a go at removing the injectors myself but conventional wisdom (Steve Heath) would say take it to a professional. Any thoughts on this? Is it worth taking to a Range Rover specialist rather than a TVR guy? Cheers

[Coefficient]

This issue sounds quite similar to my Griff though I would say that the misfire clears at closer to 3500 rpm. I’d quite like to have a go at removing the injectors myself but conventional wisdom (Steve Heath) would say take it to a professional. Any thoughts on this? Is it worth taking to a Range Rover specialist rather than a TVR guy? Cheers

[Steve_D]

That seems a rather drastic jump from having a misfire to removing injectors without (seemingly) having done any other diagnostics.
Without reading far down the original thread most seemed to be talking about ignition testing.

If you have done all of those suggested diag. then try a heat gun to first find out which cylinder(s) are mis firing.
Removing injectors is not difficult but you need to know where the problem is before pulling things apart.

Steve

[Chris71]

Coefficient said:

This issue sounds quite similar to my Griff though I would say that the misfire clears at closer to 3500 rpm. I’d quite like to have a go at removing the injectors myself but conventional wisdom (Steve Heath) would say take it to a professional. Any thoughts on this? Is it worth taking to a Range Rover specialist rather than a TVR guy? Cheers

I've got something a little similar. It never misfires, but there's a barely perceptible hesitation from around 3,500 and it seems to be a bit flat at high rpm. Engine has just come back from a rebuild and I'd been limiting myself to 3,500rpm for the first few hundred miles. It pulls like an absolute train up until that point but there's not as much top end urge as I remember. It's got a Stealth cam and would happily ram itself into the limiter previously (even though I think there's possibly more torque since the rebuild).

The car was laid up for quite a while, so I'm wondering if the (8+ years old) HT leads have degraded.

[spitfire4v8]

The negative long term trim means the ecu has detected the base mixture is rich, and is taking fuel out to weaken it off to lambda=1 (14.7afr) . If you had a misfire the trim would be positive, all other things being equal.
However this is only a snapshot at idle .. and tells you little about the mixture higher up the rev range, or once it's out of lambda control which is where you seem to be having less of an issue?
You're shooting in the dark until you can get some diagnositcs done under a variety of driving conditions really.

[TV8]

Are you still getting two black plugs as above? Personally, I would get to the bottom of that problem before doing anything else. Also, where are you if anyone offers to help or have a look?