14CUX Discovery I cold Stumble

[RGK20m3]

Starts normally hot or cold, and idle is about 1000 rpm, then drops down normally. After about 30 seconds the engine will shudder and immediately smooth back out; it will do this 4-5 times, then runs perfect. If you try to accelerate during the shudder it won’t rev or accelerate, and sometimes will stall. Starts right up again, but won’t rev above idle.

I had Rovergauge on it today while it acted up, injector pulse width didn’t change (4%/2.9ms), fuel pump indicator didn’t drop out, TPS didn’t change (6%). AFM stayed steady around 28%. Engine temperature had started to rise slightly from the 40F ambient, fuel temperature was still 40F. Fuel trim was jumping from center to -20% both banks (long term is -100% both banks, but runs great hot). Only faults are 44 (left o2) and 23 (low fuel pressure). Fuel pressure gauge on it after it was warm showed a steady 30 psi, rising to 38 with vacuum disconnected.

Any ideas?

[FridgeFreezer]

My money would be on the ignition system side of things, ignition amplifiers love to fail either when cold or when warmed up.

[RGK20m3]

I’m kinda thinking the same thing. This morning I am going to leave the fuel pressure gauge on it and hook an inductive timing light on the coil wire, that way I can see if either fluctuates while the problem occurs- which it should as the ambient in the garage is the same as yesterday.

Loss of ignition due to the amplifier would interrupt the injectors, which would explain why there is no backfiring…..just wonder why Rovergauge doesn’t show it. Only thing it shows is the idle stepper compensating for the drop in rpm,slight change in the AFM. I don’t think the tach signal drops out, just in response to the lower rpm.

[FridgeFreezer]

A misfire will also throw your O2 readings out as unburnt fuel can coat the O2 sensors and obviously leaves more oxygen in the exhaust for them to sense.

[RGK20m3]

Would make sense. Didn’t act up today, will try again tomorrow.

[RGK20m3]

Has not acted up the last 2 days; drove it a few miles with the Rovergauge on, everything seemed plausible. Will keep trying with timing light on coil wire to see if ignition is dropping out.

[RGK20m3]

Only other thing that I noticed is that while driving, the TPS was now showing zero at closed throttle; I do understand that the ECU adapts to the closed throttle reading. Is this done every key cycle? Or every time it is at idle for a period of time? I do see that while driving the target idle is achieved consistently, although the stepper will vary , sometimes zero and sometimes as high as 6%. Also both O2 sensors cycle around the center while driving but are constantly below center at idle- no idea why the long term is at -100% on both.

 I wonder if a small air leak before the O2 sensors would account for the long term trim issue? It has no misfires, has headers, no audible leak before the sensors.

Edited December 17, 2023 by RGK20m3
More thoughts

[FridgeFreezer]

The Lucas system I'm pretty sure needs the TPS calibrated to a certain voltage reading at closed throttle. Also the wires into the TPS are notorious for being fragile & breaking off right on the TPS, with replacements being reassuringly expensive.

O2 sensors should likely be a bit rich at idle, and when cruising you expect to see them continually cycling around the mid-point, they pretty much work like a switch - for regular zirconia sensors they're above 0.45v if it's rich and below 0.45v if it's lean, but it's a knife-edge response not a curve like a temperature sensor. I think the 14CUX uses titania sensors which are resistive but the reponse curve is very similar.

O2 sensors are quite fussy things - you can fracture the element with an impact, they get poisoned by the gas from some silicone sealants, they get clogged/coated with anything unburnt that goes down the pipe (oil, fuel, coolant), and of course a misfire or exhaust leak will give weird readings because it changes the amount of oxygen, which is all the sensor is measuring.

[RGK20m3]

TPS on mine is non-adjustable, and I measured the signal at closed throttle (.36v), and it moves smoothly up to 4.8v. Also confirmed on Rovergauge that it progresses smoothly from 5% to 100%. My understanding is that the ECU will adapt to any idle TPS reading below .545v on a ‘95. I had replaced it maybe 10k ago when I rebuilt the engine.

Both oxygen sensors were also replaced about 10k ago, and they both switch according to Rovergauge.

 I am leaning towards ignition but would like to confirm something before throwing parts at it. It has a Powerspark distributor with a Lucas module (on the body), along with the Lucas coil recommended. Powerspark ignition wires also. 

[AlWorms]

The TPS can be set to absolute or corrected(?) in Rovergauge. If it's in the corrected mode, it should read zero any time the throttle is closed.

An air leak before the Lamdas will cause it to *appear* lean, so the ECU will run the fuel trims rich, to try to compensate.

You may have a leaky injector, or they could be worn and letting out more fuel than normal (they are a disc-type, that do wear over time).

I upgraded to modern Bosch injectors and reduced the fuelling multiplier in the EEPROM to compensate. Prior to that, I had probably 2 leaky injectors that meant the system was leaning out the fuelling. I had rough idling after hot starts, becaue it was too rich, and ould sometime stall when selecting Drive or Reverse.

I've adjusted the fuelling multiplier so my LT trims hover around -25 and short term can be either side of 0, depending on temps, idle load, etc.

Rovergauge doesn't show the Lambda's switching, as such; more the constant cycling of the adjutments it's making. It's a pretty crude system (designed by Bosch in the 70's, licensed to Lucas later), but very effective. Rovergauge also had quite a low sample rate, because the bandwidth over the serial connection is very limited - if you turn off all the readings you don't need to see, you'll see a much improved samle rate. So it also depends on the cylinder exhaust pulse timing, lambda reaction and the sample rate of raover gauge, as to how the lambda reading will fluctuate - they're really just a guide.

You have to be careful with misfires - because the system doesn't control the ignition, OR have a cylinder disableing feature, if a misfire was occuring, a misfire will also show as a lean condition - because it's measuring the oxygen in the exhaust, and a misfire means too much fuel AND too much oxygen in the exhaust.

I'd say you have a good chance the injectors are delivering more fuel than expected, either from fuel pressure or leaking etc.

Also, because the V8 injectors are fired as 2 batches and aren't timed to anything except whatever ignition signal they start up on, a single injector does NOT feed a single cylinder - much of the fuel from any given injector ends up atomised in the plenum and feeding several cylinders... so you don't really notice one cylinder running unevenly.

It's a bit of a pain, but not truly hard to remove the injectors and fire then outside of the car and see if any leak or if they have widely varying spray patterns. For the work though, I just brought 8 new Bosch injectors and stuck them in, but I had enough symptoms I'd verified as being from excess fuel etc (like the hot start issue etc, being OK if I let it idle for a minute with the throttle open a bit etc.)

I didn't proof-read that, so I hope I said what I intended to 🙂

 

[FridgeFreezer]

@AlWorms out of curiosity / for future reference what part number of modern Bosch injector did you use?

[Bowie69]

5 minutes ago, FridgeFreezer said:

@AlWorms out of curiosity / for future reference what part number of modern Bosch injector did you use?

Every time I have ever done that with a stock ecu they just don't run quite right.... MS obviously is no problem but for a stock ecu the dead time of modern injectors are much lower so mess with the low speed fuelling. 

[RGK20m3]

I hear you on the injectors; when I rebuilt the engine I had the OE injectors serviced- one was deemed bad and was replaced. Ran great for a few days, then was hard to start hot and idled rough. Sent the injectors back and 3 were deemed bad….

 I then replaced them with a reconditioned set of Bosch (a Ford V8 application) and haven’t had any issues that I could blame on injectors for the last few years. No remapping for the new injectors, though.

 I would be happy if I could solve the cold stumble/no throttle response, the rest of my observations do not appear to affect performance. Might be addressed with an adjustable fuel pressure regulator, it seems the pressure might be a few psi high.

[FridgeFreezer]

1 hour ago, RGK20m3 said:

Might be addressed with an adjustable fuel pressure regulator, it seems the pressure might be a few psi high.

God no those are a bodge.

If you wind the fuel pressure up/down you're basically throwing everything the ECU does sideways by a fixed amount.

[AlWorms]

I used the Bosch 62354, long part number: 0280155821, because it had the best combo of matching features. Straight injection angle, narrow cone, grooves for clips and vaguely similar flow rates. I seem to recall them rates on them being better when running lower pressures (like the 14CUX), but I don't seem to have saved the flow rate charts. I think their dynamic floe rates were a bit lower than some other models, with the similar static figures.

I used these: https://www.rockauto.com/en/moreinfo.php?pk=4212903
 

I'm not saying it's ideal, but it's better than the leaking ones 😄

Prior to the change, as I say, hot start idle was funky, but it ran REALLY when going - it would rev happily to the limiter and I'd have to ease off, to make the auto change up 🤣. I've upped the rev limit and it pulls happily to 5800, with helps a little with the stupidly tall gearing with the ZF and 1.2 Transfer ratio.

Mine ran pretty good initially after fitting the Boschs, but it did send the fuel trim fairly negative, as the Lambdas adjusted them. I just changed the multiplier to get it closer to zero (I like things to be how I want them to be 😄)
I think Non-lambda cars might struggle a bit more.

Also, the later software versions have much better high idle and stepper valve control for hot and cold starts. I run a .bin based on the "Operation Pride" tune R3652, even though my Disco had a pretty late version already (R3604 - it's a 97 Model year). From memory, on really hot starts, it opens the idle stepper further, to get more air in and stabilise the idle earlier - kind of like holding high idle on mine used to help, after a hot start.

I attached a spreadsheet of Bosch injector data (note there are 3 sheets, sheet 1 has a bunch of other injectors hidden! I didn't make the spreadsheet!)

Bosch_Injector_data.xls

[AlWorms]

Oh - I fitted a switch to the fuel pump relay, so I could turn off the fuel pump to stall car the engine... which meant that when I did a hot restart, the injectors hadn't leaked into the plenum - that's how I determined the leaking injectors. Also started and idled 100% once I did that.

I wonder if doing the same would help you - turn off the pump for a few seconds when it's acting up... and if it clears up quicker, maybe that's a pointer?

Or do the same as I did - stall the engine by starving it, then just turn the key on, start straight away - if the fuel hasn't leaked in, the mixtures might be right, then It'll run fine, as it's not being accidentally choked?

[RGK20m3]

Well, it hasn’t done the cold start stumble since I switched to a non-catalyst/non-oxygen sensor map (EEPROM is a modified R3652 map), but has a new issue of stalling as I come to a stop warm. Also, occasionally the idle will not stay at the 1000 rpm as I coast down to 2-3mph. I am starting to think the idle stepper or TPS……makes it hard to drive as it is a stick.

[Bowie69]

That all sounds like a dead or unplugged MAF

[AlWorms]

Check the speed sensor is owrking too - If it doesn't know it's moving, it won't hold the idle up.

[RGK20m3]

Has done it with 3 different AFMs, and every time that I have checked it, I have a speed sensor reading on Rovergauge. Doesn’t mean either of those parts are not intermittent though. Need to get a new battery for the laptop to monitor while driving I guess.

[FridgeFreezer]

Sticky idle air valve?

[RGK20m3]

I’m thinking so, although not that old and genuine AC Delco. I might try setting the idle air bypass screw to support an 800 rpm idle with the idle air valve fully closed, and then unplug it. If it seems ok that way it would indicate a bad idle air valve. If it still acts up, either TPS or air flow meter.

[RGK20m3]

To pinpoint what component is acting up, I disconnected the speed sensor and idle air motor, then set base idle to 800- still had the issue so scratched those two off the list.

I have come to the conclusion that the problem is with the TPS; I did an ohmmeter test on a cold engine and noted the there was a decrease in resistance off the idle measurement as the throttle moved, then the normal increase as the throttle opened more. I think the refresh rate of Rovergauge is slow enough that I couldn’t see it. At any rate, I loosened it, jiggled it around and found a spot where it didn’t seem to act up. Runs fine right now.

Now I need to figure a way to install a GM or Ford TPS, because $300 is ludicrous for a $25 part. Thinking the GM might be the better option, as the V6 has the same rotation direction as the Rover.

[Bowie69]

4 hours ago, RGK20m3 said:

set base idle to 800

This is too high, book is ~600 IIRC.

[RGK20m3]

I know it is too high; I set it there because I disabled the idle air motor and speed sensor.