Limp Mode Issue

[fuzzylogic]

Hi Everyone, I know this is a well worn topic, but I have an ongoing niggle that I can't resolve. Its an 08 King Cab d40 Thai built. Starts, and runs beautifully, I've had it for 155 000 kms, same set up. My limp mode issue only occurs when I back off the gas and use the engine for compression braking. Turning the ignition off for 1 second, then re starting the ute, clears the limp mode, and car drives perfect again.

It's thrown the MAF sensor error code, so I've swapped that out, and the same symptoms occur. Otherwise runs perfect. I just don't use the engine for braking, when over about 40km/h. Around the farm, using low range, letting the ute crawl under compression down steep hills, there's no problem, no limp mode. It only occurs at speed (over about 40 km/h).

Any ideas on why this is happening? It's developed this symptom in the last 15 000 kms or so...

I can live with it, it's just annoying...

Cheers everyone

 

[Phunk]

Howe are you checking the code?

Have you scanned the ECU for all the faults?

From previous experience, limp mode is basically engine protection, it could be Electrical, could be boost pressure, could be not fuelling enough, etc.

 

[Old.Tony]

It's been my experience that codes that aren't fixed by replacing the item in question are usually caused by an electrical gremlin.

And thankfully these are really easy (and free) to fix. Clean the battery terminals, clean the negative battery lead to the engine, clean the ECU earth, clean the alternator connections.

If that doesn't fix the problem, there's a chance it's another sensor throwing the ECU data off (like the boost sensor, although we don't often hear about problems with that sensor).

Start with the electrics and see how it goes. I'd also throw a bottle of injector cleaner in the tank at the same time, it doesn't hurt, just in case it's your SCV being silly.

 

[Phunk]

It's been my experience that codes that aren't fixed by replacing the item in question are usually caused by an electrical gremlin.

And thankfully these are really easy (and free) to fix. Clean the battery terminals, clean the negative battery lead to the engine, clean the ECU earth, clean the alternator connections.

If that doesn't fix the problem, there's a chance it's another sensor throwing the ECU data off (like the boost sensor, although we don't often hear about problems with that sensor).

Start with the electrics and see how it goes. I'd also throw a bottle of injector cleaner in the tank at the same time, it doesn't hurt, just in case it's your SCV being silly.

Funny you mention it, I run a bottle of injector cleaner on every 3rd tank on all my cars. Might just be a placebo, but it makes me feel better.

 

[Old.Tony]

Funny you mention it, I run a bottle of injector cleaner on every 3rd tank on all my cars. Might just be a placebo, but it makes me feel better.

In the D40, the fuel pump has a Suction Control Valve which is quite sensitive to deposits. It causes noticeable hunting* at idle and causes wear with a good chance of failure. They're about $240 to replace, two torx bolts and one electrical connector and I can't do any of it with uncle Arthur now firmly entrenched in both hands. Quite often, fuel injector cleaner resolves this issue.

* Note: the ECU does modulate the fuel rail pressure at idle and monitors the results with the heated oxygen sensor in the exhaust to determine if the system is functioning normally, but this modulation is too slight for us to detect. The "hunting" is quite a noticeable thing, like someone's going to 5% throttle and backing off again constantly.

 

[fuzzylogic]

In the D40, the fuel pump has a Suction Control Valve which is quite sensitive to deposits. It causes noticeable hunting* at idle and causes wear with a good chance of failure. They're about $240 to replace, two torx bolts and one electrical connector and I can't do any of it with uncle Arthur now firmly entrenched in both hands. Quite often, fuel injector cleaner resolves this issue.

* Note: the ECU does modulate the fuel rail pressure at idle and monitors the results with the heated oxygen sensor in the exhaust to determine if the system is functioning normally, but this modulation is too slight for us to detect. The "hunting" is quite a noticeable thing, like someone's going to 5% throttle and backing off again constantly.

ThanksTony, the SCV was on my radar.

Howe are you checking the code?

Have you scanned the ECU for all the faults?

From previous experience, limp mode is basically engine protection, it could be Electrical, could be boost pressure, could be not fuelling enough, etc.

 

[fuzzylogic]

I checked diagnostics with my mechanics OBD thingy, it was a little while back, so I don't remember the code number, but it said MAF.

I've chucked in injector cleaner a few times in the last few years, always thought it was a gimmick, but convinced to try by Old Tony. Nothing to lose really.

I'll try the electrical gremlin investigation I suppose, but my instinct was the SCV may be the culprit. It's such a specific way the limp mode is triggered, normal, hard, soft and extreme driving doesn't worry it, just compression braking.

It idles like a Swiss watch at all times except for initial start up on really cold mornings, where it does 'hunt' a little. I always thought that was just because of the cold.

If I do the electrical gremlin thing, and no change, is there any type of test I can do on the SCV, to help with diagnosis?

Last oil change, same oil, same quantity, for the first time on start up, the oil light started flashing, and the odometer changed from numbers to say 'low oil'... it went away after 30 secs, I'd just never seen it before, quite trick...

Thanks fellas

 

[Old.Tony]

Usually the SCV is either dirty or scored (my old one was very obviously scored). Removing it will give you that answer.

When you're compression braking, engine load should be zero, and the fuel injectors should be turned off (I've watched mine do this on numerous occasions). The SCV should still be operating, because the fuel pump - even though the injectors are not commanded to open - should still be operating. It might still be the SCV - but it's an unusual manifestation of the problem.

 

[Phunk]

In the D40, the fuel pump has a Suction Control Valve which is quite sensitive to deposits. It causes noticeable hunting* at idle and causes wear with a good chance of failure. They're about $240 to replace, two torx bolts and one electrical connector and I can't do any of it with uncle Arthur now firmly entrenched in both hands. Quite often, fuel injector cleaner resolves this issue.

* Note: the ECU does modulate the fuel rail pressure at idle and monitors the results with the heated oxygen sensor in the exhaust to determine if the system is functioning normally, but this modulation is too slight for us to detect. The "hunting" is quite a noticeable thing, like someone's going to 5% throttle and backing off again constantly.

I've seen a lot about this SCV since grabbing Navara, and I can vouch for noticing a slight surging on idle but it's sporadic.. doesn't happen every time. It's definitely dropped since introducing my injector clean regime!

What are your thoughts on Fuel Doctor? I've been contemplating giving it a go.

 

[Phunk]

I checked diagnostics with my mechanics OBD thingy, it was a little while back, so I don't remember the code number, but it said MAF.

It idles like a Swiss watch at all times except for initial start up on really cold mornings, where it does 'hunt' a little. I always thought that was just because of the cold.

The fact that it said MAF as an error and idle is rough when cold but stops when it's warmed up, you took the words out of my mouth.

Majority of sensors have a heating element to get them up to speed and get correct readings. If one of the sensors are playing up, it would give this type of symptom.

Next time it's running rough on startup, unplug the MAF and see if it makes any difference. (The MAF is the sensor connected to your airbox piping)

 

[Billy54]

I have a 2011 D40 and the only issue has been an SCV which got replaced a few years back since then i add fuel doctor to every tank and runs sweet as a nut [touch wood] Back in the day when you could go on a road trip i put 300mm in to start then 100mm every tank, felt the difference saw the klms per tank increase dramatically. Buy it at supercheap in the 1lt bottle when on special.

 

[Old.Tony]

I've seen a lot about this SCV since grabbing Navara, and I can vouch for noticing a slight surging on idle but it's sporadic.. doesn't happen every time. It's definitely dropped since introducing my injector clean regime!

What are your thoughts on Fuel Doctor? I've been contemplating giving it a go.

They're all similar. They'll contain things like naptha and other agents to act as solvents and allow combustion. My personal preference is Liqui Moly Fuel System Treatment, but that's only because it's the one I picked first and it worked so I stuck with it. I have checked other cans out, they all seem to contain the same sort of thing.

 

[Phunk]

They're all similar. They'll contain things like naptha and other agents to act as solvents and allow combustion. My personal preference is Liqui Moly Fuel System Treatment, but that's only because it's the one I picked first and it worked so I stuck with it. I have checked other cans out, they all seem to contain the same sort of thing.

Years and years ago I had a rep from Nulon come out and give us a demo on their products, it genuinely surprised me on how good they were. I've stuck with Nulon for all these years, but the price keeps creeping up!