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TD5 in limp mode after starting

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Bit of a strange one: recently had a Disco TD5 come in with poor starting. Glow plugs had been replaced last winter, but didn't make much difference. So I decided to do the injector seals, and they turned out to be in pretty poor shape. Fuel filter, check valve and bleeding valve also replaced.

Car started fine when he left, but after a few days it became worse again. And with an interesting twist: sometimes the car will start fine, but idle a bit low (700 instead of 750rpm) and not respond to the throttle at all. If left idling for a minute or so, it will clear, raise idle a bit and drive fine all day long. I hooked up the Rovacom, no fault codes and all values seem correct. Even when the idle is too low (it does show a difference between actual and desired idle). TPS is responding when you press the pedal, but nothing happens. Injectors are all well balanced, MAF reading 50ish. Only strange reading is the MAP, which is reading low, 30 to 40 with ambient pressure at idle. It does go up with revs and load once you're driving, but remains too low. Though the car doesn't feel especially sluggish. Little bit of oil in the red connector, cleaning makes no difference.

It looks like an electronic problem, but strange it started shortly after doing the injector seals. Not sure where to look next...

Filip

 

 

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Sounds like the maf is getting tired to me.

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Hi, Are you able to check the fuel pressure with a gague at the FPR. Need 4 bar. This will rule out the fuel pump.

Regards,

Griff

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Looks like my reply from yesterday didn't get through...

MAF has been unplugged, but doesn't make a difference. Value is normal at 50ish at idle and rising when driving.

Haven't checked fuel pressure yet, certainly worth a try. I would expect low fuel pressure to be a problem at higher load/revs, which isn't the case, once it gets through the limp mode in idle.

It really feels like an electronic problem, but with no codes hard to prove. The injector seals are the last change made, but I can't see how could they could cause a low idle and no reponse to the throttle. Feel free to correct me!

Filip

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3 minutes ago, Escape said:

Haven't checked fuel pressure yet, certainly worth a try. I would expect low fuel pressure to be a problem at higher load/revs, which isn't the case, once it gets through the limp mode in idle. 

Fuel flow would cause more issues at high revs. Fuel pressure, if lower than spec, would mean less fuel being delivered during the injector opening time.

So yes, certainly, low fuel pressure will affect running across the range.

if fuel flow was the issue, I'd agree, it would only/mostly have affect at higher rpm.

Your comments about the maf, and when unplugged, is getting a little confusing...

 

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Sorry about the confusion. As said, I typed a reply yesterday evening, but it looks like that didn't get through.

We tried unplugging the MAF, but that didn't make a difference. With it connected, I get normal values on the Rovacom. So I don't think it's the MAF.

Fuel pressure is something I need to check.

Filip 

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When i got my  TD5 a few years ago i had a similar problem and it turned out to be the fly by wire connection between the accelerator pedal and the ECU in the engine.  All fine following replacement of the pedal.  Might be worth looking at if all else fails 

Hugh

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I had an issue with the throttle pedal. Even after I replaced it it still caused a problem. I had to get my local independent to put it on the computer but it still baffled him until our of frustration he changed the throttle pedal and it worked fine and has done since.

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I did look at the throttle pedal, but even when the engine isn't responding, the voltage on all three pots is correct. So it looks like the ECU just doesn't want to let the engine rev. It does throw a 'driver demand' error when you press the pedal while it's in limp mode.

Soince posting, I've gone through the entire fuel system again (with Elbekko), replaced injector seals again, rechecked filter, housing and bleed valves and verified both the LP and HP side of the pump are working. I've ordered a new MAP/MAT sensor, as that's the only problem I can identify.

Filip

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Filip

I had the AA check the voltages on the pedal and all registered fine. On one occasion they sprayed solvent cleander on the ECU plug and ECU which cured the problem for a couple of hours.

 

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Did a few more checks, unfortunatley still haven't solved the problem.

- we put in a pedal from another D2, made no difference

- replaced the fuel pressure regulator. It was sweating a bit, but not leaking. As it was the last part in the fuel system we hadn't replaced, it was worth a shot. Car started easily after bleeding, all seemd well. Then on the next attempt, easy start but back to limp mode for about a minute before it would respond to throttle.

- an experienced (now retired) LR mechanic had a look, checked all my work, could find nothing wrong and is also at a loss...

Filip

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@Griff, good tip, I'll have a look there.

The car has since been to another independant LR mechanic, but he couldn't find anything wrong either...

Filip

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I seem to recall that the TD5 will not rev for around 10 seconds if the MAF is disconnected. Normal service resumed once it decides there is no MAF and resorts to using a stock table from its memory.

I suggest a new / known good MAF to start with and then check the wire continuity down to the plug to the ECU. Had a friend who had MAF wire broken at ecu socket. (As well as air temp sensor wire)

Truck ran but because of temp sensor was under powered. The only thing that really stops them running is a lack of Crank sensor.... Other than that they a have a lot of redundancy options built in.

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We'll take another look at the MAF. The readings are normal, including when it's in limp moden after starting (ca50kg/hr at 700rpm). Disconnecting the MAF didn't make a difference either (to the running/starting that is, obviously the readings did change), but if it's in the wiring, that is to be expected.

Filip

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