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Discussion Starter #1 (Edited)
I know this has been discussed many times but I cannot find direction from the currently available posts. Also, many of those older posts do not have follow up citing solutions to their problems.

I'm attempting to troubleshoot a running/starting issue on my 94 Classic. Recently the truck has not been starting as strongly as normal from cold start. By strongly I mean it starts at a low idle, runs rough and after a few seconds the idle will come up to normal slowly and continues to run rough.

If driven and attempting restart when hot the engine will start, then die. It will repeat this for a few times before stumbling to a normal idle speed but still running rough. When driving the engine has good performance and feels strong. When coming to a stop the engine almost dies but doesn't. If I sit long enough the idle will attempt to recover but is still low and rough.

Here is the list of things I've replaced:

  • distributor cap
  • distributor rotor(genuine bonded type)
  • plug wires
  • spark plugs(OE Champion RN11YC4)
  • Coil(OE Bosch)
  • O2 Sensors
  • fuel filter
  • Swapped in known good MAF

I have tested the fuel rail temp sensor and ECU coolant temp sensor and they are reading normal and changing resistance as temps change correctly. I don't know the age/status of the fuel pump or the pressure regulator. I'm not getting a rich fuel smell from the exhaust and there is no check engine light or code on the OBD readout. I have checked for vacuum leaks but cannot find any.

Last week the truck was running perfect. Better than it ever has. It's been slowly degrading over the past few days.

Appreciate any and all ideas.

Thanks
 

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1989 Range Rover Classic
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I still say ignition amplifier. It's not on your list of recently replaced, and it's notoriously finicky. Rough running, unburnt fuel smell, and occasionally nearly dying sound like random misfires, which is what a ignition amp on the fritz would cause.
 

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The idle overall is implimented by the IACV valve which you need to verify.

Try this it's simple to assess at least if it's doing something.

When stone cold, turn on ignition but don't crank it, then turn off again. It should now have set the IACV to fairly open ready to start with, initially, a fast idle.
Now pull the multi pin plug from the IACV to isolate it from the ECU, then start it.

What you should get is a very high idle speed about 2000 rpm that you have no means of controlling.

This would show you the idle circuit control is at least functional. If no difference, then it's not working.

To get back to normal, just switch off, plug in and restart.

The IACV is just an air valve that bypasses the throttle plate, a hole with a cone to plug it that's moved in and out by step motor in response to call from ECU.
Simple logic just steps in if above target (report from pulse via wire to coil) or pulls the cone out if below target.

If not working, the ECU calls but nothing happens.
 

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Discussion Starter #4
Since posting this I have performed a few troubleshooting steps.

I have fully troubleshot the IACV component. First by performing the test described above. The results were as expected for an operating IACV circuit. Then, by swapping it out with a good used part I had on hand. I'm very certain that is not the issue.

Next I installed a replacement amplifier. This did not change anything. It is still running rough at idle and has trouble restarting.

I used a timing light to verify consistent spark at every cylinder. All seems to be sparking normally.

At this point I'm thinking fuel. I pulled the cargo area carpet and insulation to expose the fuel pump. My goal here was to verify the connector is seated properly and free of corrosion. I realize there are a lot of electrical components between the ECU and the pump that could fail and I will troubleshoot those if needed. I'm starting simple and working up. The plug was set and very clean. It looks as it the pump was replaced recently. Also, the engine runs great under high demand. If the pump were failing I'd suspect the pump to starve the engine under high load.

Next I moved to the fuel pressure regulator. With the engine running I pulled the vacuum line and listened for a change. No change!! Running as rough as before. It exhibits the same exact characteristics during start up when disconnected.

I am now suspecting FPR. I don't have a way to test fuel pressure. Can anyone confirm running characteristics with the FPR vacuum disconnected?
 

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1970-1995 Range Rover Classic
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One simple and quick test to be done on the FPR is to see if the vacuum line holds vacuum or pressure. If it's leaking, it's definitely bad.
 

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Does the heater "select" slider bring the idle up when you push it across to the left (the one that selects heat or air con) ?

Even with a cylinder sporadically out on mine the IACV circuit could normalize the idle speed, it responded fast enough for drop out so as to need no interference with throttle to keep it running.
 

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Thinking for the above is, if you're happy with your IACV verification, then the heater control offers a "manual" channel to prompt it.

The cold start is also in effect a manual channel too as it has zero rpm, cold signaling temp sensors, to make ECU pull the IACV to open in readiness to start.

But when it's running I thought it also wanted to see road speed below threshold and TPS closed, so if either didn't fulfill then the ECU wouldn't pick up idle control.

More thinking out loud for discussion than certainty.
 

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Discussion Starter #8
Yes, heater selection prompts idle change. This was also verified by turning on front/rear heated screens as per service manual verification. Pretty sure IACV is working correctly.
 

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Discussion Starter #9
I performed a test of the IACV. Started the truck and got the engine warm. Shut it off then restarted. It exhibited the low rough idle we've come to be familiar with. While in this state I unplugged the IACV effectively "freezing" it in its position. Slowly the idle started to rise to about 1500 rpm. What this tells me is that the IACV is opening and trying to allow enough air to raise idle but other factors are preventing this. I believe it might be fuel. Over/under fueling caused by a stuck or inoperative FPR may induce this characteristic. That's my thoughts. Open for discussion.
 

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Certainly to close off some components the only way would be to verify as to their accuracy, fuel pressure being one of them. Can you rent a pressure guage from motor factor store to check that, or potentially change the regulator. Interestingly, I've never had a confirmed as failed regulator in my hands, they seem very reliable.

I was thinking along the lines of TPS inconsistencies as it directly alters fuelling by the ECU translation into injector duration adjustment. Have you tested the TPS for linearity? As I see it this can alter pulse length of injectors but the MAF sees no change as air volume didn't change, if a problem exists there it would cause system confusion.

Rover 14CUX Hot Wire Mass Flow EFI: Service and Troubleshooting presumably you've seen the attached link as it gives good overview of system.
 

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Discussion Starter #11
Yes, I agree. My local parts stores rent generic test kits. Can't seem to find one with the correct fittings. Might have to make my own setup. I ordered a replacement FPR. I agree, they are very reliable but there is potential for them to fail given how they operate. The truck has 192k miles and I don't think it has ever been replaced. I planned to replace it anyway as a form of preventative maintenance. It should be here tomorrow. If it doesn't make a difference I can rule it out.

I've read that article about a dozen times over the years. Very comprehensive and helpful. If the TPS was faulty I thought I'd notice other running characteristics but it's an easy free test that could rule it out. I'll report my findings.
 

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Discussion Starter #12
Update

The source of one of the two issues might have been found.

Reading through the 14CUX troubleshooting guide link posted above (thank you RRLondon) I started going through all the connections easily accessed in the engine bay and under the truck with the goal of systematically checking them off. I came to the ECU rpm step down resistor and when I removed it from the harness I found the terminals heavily corroded and green. After cleaning the spade terminals on the resistor and using electrical cleaner on the harness connectors I tested the resistor and it's reading about 6700 ohms. Reinstalled the resistor and the engine is no longer stumbling at hot restart. I get the 1200-1500 rpm flair at startup and now it settles down at 650-700 rpm. The idle quality still isn't great but starting strong is a huge improvement. Next test will be to drive the truck and see how it acts dynamically on the road.

I also received the FPR today. I'll install and see if anything changes. More info to come.
 

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Discussion Starter #13
Installed FPR and test drive. Truck is running just as poor as before. Performed test for TPS and it's working as it should.

Idle does not recover when coming to a stop. Idle is rough and erratic.

Not sure where to go next.
 

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I can't remember if you've posted any details about engine mechanical condition or if it's rebuilt etc.

Certainly I'd want to know compression, condition of cam etc if I stood in front of it trying to diagnose.

A quick check, open oil fill cap, sucure rubber glove with zip tie round opening, run engine. What does the glove do?
 

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Discussion Starter #15
I installed a replacement engine a few months back. It's a 3.9 that came out of a running/driving truck. The compression is good. I stripped the engine down to a long block and resealed, cleaned and refurbished all parts before going back together. The cam looked great and a new timing set was fitted. I did not fit new head gaskets, should have done but oh well. Before the issues noted in this post started, the truck was running perfect. Seriously good. That only lasted for about a week.

I'll do the rubber glove test over the weekend and report findings.
 

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It's certainly puzzling, also frustrating too.

As an indication, mine even increases revs if I remove the dipstick and then the IACV pulls it back to aim point. So it's operating a vacuum within crankcase normally as I interpret it.

Be interesting to see if yours is running pressure or vacuum with the glove check.
 

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Discussion Starter #17
Performed the glove test. The glove inflated, meaning there's some blow-by which is to be expected for a vehicle of this age and mileage. I inspected the PCV lines and the flame trap on the passenger valve cover is super clean still from when I installed the engine. The rest of the lines were clear. The T-junction was not. The small metered portion that is under vacuum when the engine is running was completely clogged. I removed the T and the brass metered port and cleaned them and assembled everything.

Now the glove shows vacuum when engine is running.
 

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Did clean the left hand rocker cover vent and put in a new filter? that hole in the cover is small. Mine was bunged up
 

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Performed the glove test. The glove inflated, meaning there's some blow-by which is to be expected for a vehicle of this age and mileage. I inspected the PCV lines and the flame trap on the passenger valve cover is super clean still from when I installed the engine. The rest of the lines were clear. The T-junction was not. The small metered portion that is under vacuum when the engine is running was completely clogged. I removed the T and the brass metered port and cleaned them and assembled everything.

Now the glove shows vacuum when engine is running.
It'll be interesting to see if it offers any changes to the running now. Certainly they seem to respond to very small differences in airflow so it can't hurt to correct such problems.

I know crankcase pressure will ultimately come from blow by (top compression ring not holding full pressure) but a significant accumulation in there would appear to ultimately pass back up into combustion and give some level of uncontrolled air scource sporadically.
 
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