The engine sometimes dies at when coming to a stop.. before it was at a high idle of 1200 and then slowed down after a second to 700 or so rpm. It does not seem to do have a problem when cold.
This is a common *Feature of the 14CUX system.
*Everything below is MY observation and experience; I have not studied the 14CUX operating manual (please reply to this thread with the full logic tree and description if you actually have it!
You can clean the injectors till the cow comes home.. it may help with mileage but it will not likely help with this behavior.
High altitude opinion: the Idle control servo reacts WAY TOO SLOWLY for the 525-1800rpm range 'goings on' with this engine.
Even when working perfectly, the pre-GEMS engine, with manual transmission (R380) is prone to deep RPM swings when below ~1800RPM... this behavior led me to do the Electric Power Steering Pump Conversion (found in painful detail here on Discoweb) to resolve city/parallel-parking Engine Dying Syndrome [CPEDS]
Partners in crime for this *Feature [hopefully others will chime in to correct me or otherwise add to the description]
- Idle Control Servo,
- O2 [Lamda] Sensors
- TPS [Throttle Position Sensor]
- Distributor Vacuum Advance
- Speed Sensor
- Fuel Temp Sensor
- Coolant Temp Sensor
- 14CUX ECU
- Hot-running vapor-lock (vapor/air in the fuel rail)
Cleaning, calibrating, and replacing some combination of these parts helped [mostly] resolve this *Feature for me.
The throttle position sensor, in my case, was heavily worn in the just off-idle--to--mid-range throttle position.. the signal would be nominal at idle but would immediately go asymmetric off idle until about half throttle; this caused the computer to jump all over the place. It was exasperated by a wrongly wired O2 sensor (Port Side) that was providing bad feedback to the 14CUX. When combined with a very slow-to-respond idle servo/valve, I ended up with the following situation:
IF the Disco was rolling (and giving an above-zero speed input to the 14CUX) the engine would die when coming off-throttle at lower rpm (say, from 2500rpm or so, down to idle, when the engine was warm). This happened ALL the time when rolling to a stop at a light.
Note: the TPS for the 14CUX is STUPIDLY EXPENSIVE. There is a conversion kit to use a "common" TPS (I think it's the one used with the GEMS system) that is much cheaper.. though the conversion kit itself is ~$175 or so. *If you have a desktop mill you can make it for about 25¢ in brass and $25 for the TPS. This conversion kit includes a cover/alignment plate that is NOT likely to work for you (at least it did not for me) because the TPS has to be physically calibrated (rotated clockwise/counter-clockwise) to get you into the specification your 14CUX wants to see.
When the engine is "decelerating" or being used to ~jake-brake [when the engine is a good ways above idle AND the TPS is at 0% AND there is a speed-input] ... the computer chops the injectors off to "improve economy" (aka: avoid backfiring) then, as the RPM falls into the range below ~1500rpm or so, the idle mapping kicks in and takes over...with the idle servo/valve trying hard to keep the engine from dying (by falling below ~1000rpm). It is not until the speed sensor tells the 14CUX "we are not moving" that the 14CUX then drops the idle from ~1000-1200 down to it's base of ~525ish..and it does this in large steps (normally landing around 700rpm before a slow ratchet down to ~525). This whole process is crude at best.. and challenging to dial in.
My old distributor also failed to provide it's full [any] range. I installed a new distributor and amplifier (I won't name or recommend the brand I went with. The quality was complete shit and the support was garbage...you can tell I'm not a fan). It worked after making some repairs and changes to it.
There are two idle management behaviors that I observe, one for stationary and one for speed-indicated (rolling) situations. Both are also modified based upon temperature.
- Idle/not-moving RPM is ~525rpm (the idle servo/valve struggles to keep it stable, so don't think you're a lesser man because your warm-engine idle cycles wildly from 450-600).
- Idle while moving: this seems to be in the 700-1200 rpm range (engine warm). During deceleration, you should see the engine bounce off the 1000-1200rpm mark....and then as the wagon rolls to a complete stop the system should drop the rpm down (likely in big chunky steps) to ~525rpm or thereabouts.
Regarding a hot engine that is *fully heat soaked*: I think the fuel pressure in the rail is TOO LOW and it experiences vapor lock like behavior after highway running for a long period of time. I think a slightly higher fuel pressure would help avoid low rpm (decelerating rpm) engine dying and hard starting in this case. I am tempted to crush the regulator to get the pressure up to ~45-50psi (this was how we did it at Dinan, back in the day). This should be a safe range for healthy injectors and it would increase the boiling temp by about. I don't know what the actual boiling point (or really, the vapor forming point) is for gasoline but I know if varies *wildly* based upon composition. It's fair to say that at my current ~30psi reading (I have a calibrated gauge permanently mounted on the fuel rail inlet), the boiling point is around 200º [your results will vary] and each 5psi increase in pressure pushes that boiling temp up about 5ºF.. So taking it to ~50psi will gain about 20º of headroom for that vapor lock issue. I am also tempted to insulate the rails, put heatsinks on the rails, and ventilate the hood above the exhaust manifolds.
I am surely missing something or giving mis-ordered info.. I commented about all this in an earlier post when I was pulling my hair out trying to fix it.
[*Feature: I use the Lucas [POD] definition herein]
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