Suggestions Please - Rough Idle and Misfire under load

I'm really struggling with an ongoing issue on my 1997 VVC.

The car has suddenly developed a lumpy idle, and is misfiring under load. The plugs leads and coils were changed by the previous owner around 8 months ago, and I've since upgraded the throttle body along with cleaning the IACV and replacing the IAT and Coolant temperature sensors.

I purchased the PScan unit which revealed the faulty temperature sensors (they were recording default values regardless of external conditions) but along with a trusted mechanic we've still been unable to nail the root cause of the misfire.

Does anyone have any suggestions - I've attached my logged data from a test drive yesterday in the hope that someone might spot something I've missed.

The readings you are getting at face value are exactly what I had when I had this problem (I have a Jan 1998 VVC).

I pulled your attachment down, reformatted it (ie delete the blank cells) so that data was properly aligned and then checked against my own extensive readings over the last 2 years.

You need to get the oil temp to 85 C or above and then let the engine 'idle' for 5 minutes. Download the data again during this hot idle. That way we can properly assess the data at full operating temperature.

The stepper should be at 26 steps at hot idle (880 rpm). Even from your own readings, I see that when you are at circa 880 RPM (which is rare), the stepper reading is circa 60 steps, not 26 steps. The throttle at 4 degrees open which is a relative reading to the throttle position when the engine is off, if correctly calibrated, is spot on at idle (but...). However the throttle valve status is showing open when it should be showing closed + the throttle angle is stuck at 4 degrees when it should vary as the accelerator is pressed so something is a miss here.

Vacuum pressure should be around 32kpa at hot idle (880 rpm) but the high revs are distorting the reading.

With readings taken at oil temp 85 C + and a steady idle for 5 min, should give us a better chance at finding the fault.

Thanks Julian, this is really helpful.

I should have added that the throttle body is now the 52mm version - would this affect the expected values?

No. I have a 52mm throttle body as well.

Updated values attached - I ran the PScan from startup right through, but it should give a good sample at the temperatures you suggested.

I ran the IACV reset afterwards to see if this made any difference, but it's still sitting in the high 30s.

(p.s - tips on removing the blanks from the data would be great - I'm fairly competent with Excel but I've been unable to get this into a more user friendly format!)

Revs at idle are bouncing around somewhat (mine sits at 875), vacuum at circa 39pa is a little low (mine sits at circa 33pa but gut feel is that your vacuum pipes are not leaking) and the no of steps on mime is 26/27 and yours sits at circa 40 steps. Other readings close to mine including the 4 degree throttle opening at idle.

First physically check the IACV pipe from the throttle body is not leaking in air. Check no vacuum pipe leaks but this can be difficult unless you have kit to do a smoke test. Clean the IACV plunger - I used petrol.

Within **Pscan, there are instructions as to how to adjust the no of steps on the IACV. It is an iterative process which you do slowly to allow the engine to adjust to the new settings. From memory, the adjusting (which is done at IDLE) should be carried out with the vacuum pipe OFF and once in equilibrium the vacuum pipe is reattached. I also reset the TPS at the end of the process and double checked via Pscan the readings to ensure all within spec

Everything is in equilibrium at idle when 1. RPM is steady at 875 rpm 2. Butterfly valve is physically open at 4 degrees when accelerator not pressed and the throttle position sensor is telling the ECU that the throttle is 4 degrees open 3. Stepper motor is 26 steps away from closed/engine off position (it is allowing additional air to bypass the butterfly valve - the higher the number of steps the more air is bypassing and of course, the more air that is passing the more fuel is going to the engine - the IACV is making constant adjustments to stabilise the idle).

Note that ALL adjustments are done at IDLE when the engine is HOT (by hot I mean to go on a fast run to get the engine oil up - 85 degrees). At this point, the water temp will be around 95-100 degrees.

* Rover say do not play with the idle screw and it it covered by an anti-tamper cap (which you will have to remove) on basis that the throttle body is calibrated during manufacture such that the butterfly valve is sitting at 4 degrees when the accelerator is not depressed . I did adjust it as it was the only way to get the revs steady at 875rpm and no of steps at 26 and it has made the big difference. The key question is why it got off-kilter in the first place, an unanswered question.
** Often people say that this adjustment can only be done on Testbook. What they forget is that in many parts of the world, finding Testbook is impossible and it will get more difficult as time goes on as this kit goes back to the late 90's. The only option left are the likes of Pscan where utilising all the data it produces allows you to get the engine back into equilibrium - in a way going back to the old days when one made carburettor adjustments to get idling /fuelling before the advent of 'throwaway and replace with new' smart sensors (the shortage of car parts across the automotive sector, including on newish cars, is going to force us to go back to the old days of fixing what we have).

Thanks Julian, I'll give this a try after work.

I did clean the IACV with carb cleaner (it was dirty, but not excessively) but didn't check the vacuum hose for splits/holes so I'll look at this again. Update to follow!

I agree with Julian that you should adjust the idle screw. I have done this many times on various cars and it is not the nono as commonly thought.
wear over time(or swapping throttle bodies) means that it is necessary.
It is obviously easier with pscan or testbook but with patience it is doable. I have a post on the procedure somewhere on the forum.

Basically you need to adjust the idle screw out a 1/4 turn at a time and then wait for the idle to stabilise, before repeating. ~you will hear the step change in the idle speed until you get it down to the correct number.
idle adjust

Took the car for a decent drive after work today to get the temperatures up, and there seems to have been a bit of progress.

I've adjusted the idle screw and managed to get the stepper to the desired position, but the idle still seems to be hunting slightly. Strangely enough, after a few minutes being fairly steady it started to creep up again and stayed there (as seen in the data). A plip of the throttle seemed to settle things back down again but it still feels lumpy.

In short, better but the miss is still there.

*EDIT* on reviewing the values again, the other potential oddity is that the o2 sensor heater is active throughout despite having been on a fairly spirited drive. Does this match up with your datasets or could there be something amiss here?

02 Sensor heater stays active. If not, you have a problem.

You will find that it will take several drives and adjustments to get it spot on. The engine's ECU is trying to find an equilibrium. Do a TPS reset again when the engine is hot, go on a decent drive, check the readings again and readjust the no of steps to 26 as necessary. You do not want to go below 26 steps and the no of steps really needs to be a steady reading, whether it is 26 or say 32, when the oil is above 85 degrees.

It takes time for this equilibrium to be found so be patient.

One thing I did do for the hell of it was used Pscan to reset the ECU settings to factory - it is in the Pscan menu - i.e The ECU forgets all its previous adaptions and starts to readapt again - it should not be necessary as the ECU is constantly readapting.

I'll give that a go later.

Could the stepper be the issue? I can hear a faint noise when the ignition is switched off as it moves back into position, but is there a chance it would the source of the hunting/hesitation? It's almost as if the heavy acceleration is catching the injection out - a sudden rush of air with insufficient fuel to ignite.

The stepper seems to be working based on what I see - the hesitancy could be due to over fuelling if too much air (too many steps and butterfly over 4 degrees open; if it is, you will smell it in the exhaust fumes. In my case there was hesitancy and then like a delayed turbo the engine would suddenly pick up and over rev. It was very noticeable when accelerating up hills from a slow start.