For a while now my 08 JKU has been giving bad EGR like symptoms and very occasionally throwing P0300 but nothing else. It has 85000km on the clock.

When accelerating from no load (coasting, idling) and stepping hard on the accelerator the engine will falter (more than a flat spot) between 1500 and 2000 rpm. It gets quite violent if you don't back off. Sometimes the CEL will come on displaying P0300 (multiple cylinder misfire) but only that code, and only sometimes.

If you accelerate with the throttle already partly depressed this doesn't happen, and above about 2200rpm it runs perfectly. Idle is slightly erratic. Otherwise it seems to have good power and no other problems but suspiciously good fuel economy - between 11 and 12 l/100km, have had it to 10.7 on a long journey.

Could it be the EGR is stuck open? But all the problems I've read about with the EGR have it throwing a P040* code and mine isn't. Should I replace the EGR anyway or look at MAP sensor/forward O2s/fuel pressure regulator? It's had new plugs/leads in a recent service.

Many thanks for any suggestions

 

hmm, so I take it no one has had this problem...

 

I guess anything is possible, but the EGR is thoroughly monitored.
Its position is physically monitored, the circuits for it are electrically monitored, and changes in engine behavior with it open and closed are monitored.
Sometimes it will open and close the valve just to check for the expected difference in engine roughness.

I would say if you have no EGR codes, the EGR valve is functioning as expected.

 

thought i was on to something there. What might be the best things to check now then?

 

Plugs, coils, plug wires, fuel pressure, contaminated fuel, bad valves, vacuum leak, just to name a few.

Multiple cylinder misfire, (P0300), has a long list of possible causes.
Engine Mechanical is one item on the list. It has its own long list of things to check.

 

yeah I realise p0300 is very vague, but I was hoping as the plugs and leads have been changed and with the symptoms I've described I can start pin pointing it a bit more

 

Well I just made up a couple of blanking plates, removed the steel pipe and blocked the holes off. I don't think it made a difference - still stuttering in the 1500-2000 range, slightly rough idle. Interestingly no CEL light though, and it definitely wasn't worse. In fact it may have behaved better (more 1500-2000 power and slightly smoother), but I'm putting it down to placebo. But could the ECU take a while to "re-learn" things and it may improve?

I'll refit the pipe when it cools down as my wife's taking it in to the mechanic in town for an air con re-gas and he'll run a diagnostic machine on it to see if it tells him anything (don't think his gives live readings though so I don't think it'll get anyway). If he doesn't pull anything I might refit the plates and use it for a week or two and see if it improves. I won't do any damage will I?

 

Blocking the EGR without reprogramming the ECU may lead to spark knock, or pinging, under load. The ECU will retard the timing to reduce the pining, but it may be worse than it can compensate for.
(Just a theory. Listen carefully, and be prepared to hook it back up.)

 

Sounds like a vacuum leak.

 

Also check your Left exhaust manifold... if it is cracked it could be blowing hot exhaust gas on the ignition pack, causing it to overheat the electronics inside... that will cause a misfire as well good luck