Misfire driving me mad!!

Message

BigM · #1 Post by **BigM** » Wed Sep 23, 2009 1:50 pm

Here's a challenge for the online pool of knowledge

I have a Jeep Grand Cherokee 4.7 1999 with a prins multipoint LPG system and I'm ready to admit defeat at trying on my own to cure this! Also this may not even be an LPG problem as it seems to happen on petrol also, but help would be greatly appreciated....

My car is slightly misfiring when at motorway speed (70-80) and only when under load.... the misfire gets worse when going up hill or switching the air con on or having the car full of people.
I have changed the spark plugs twice (now have a cooler set in) and also changed the coils (changed 4 and no difference, so changed the other 4 and still no difference).
I have had it plugged into a Jeep diagnostic unit and no faults were shown.
If I have the car under a very small load the misfire seems to be about once every second, but if I then put my foot down it misfires loads.
Not completely sure, but seems to be better when the car is first started.

As I said earlier it seems to happen on either gas or petrol so I'm pressuming it won't be anything to do with the fuel system, but did wonder if any part of my fitted LPG system could cause this?
Will post this on the V8 section as well in case someone misses it here

Many thanks in advance to anybody that can help me.

Mark

SAD199 · #2 Post by **SAD199** » Wed Sep 23, 2009 2:31 pm

If it's misfiring on petrol and gas and, given you've covered the things I would have done, I'm wondering if it's a mechanical thing like the valves. Have you had them check/compression test and so on. It can't an injector as it's doing it on both fuels, so I'm also wondering about the cps going intermittent? How many miles it done on gas?

BigM · #3 Post by **BigM** » Wed Sep 23, 2009 2:39 pm

Hi

The car has done a total of 54,000mls, the last 15,000 of them being on gas.
I have not compression tested yet....I hope it's not as serious as that!!

rossko · #4 Post by **rossko** » Wed Sep 23, 2009 3:34 pm

Get a "real mechanic" who knows how to interpret a manifold vacuum gauge (these people are rare to non-existent at main dealers), they'd be able to spot any issues like broken valve springs.

BigM · #5 Post by **BigM** » Wed Sep 23, 2009 5:15 pm

Therefore could this be a problem with a stuck valve....maybe caused by the lack of valve lubricant due to running on gas?
Didn't really imagine it could be anything this bad.
Do you think it would be worth getting it put on a rolling road to try and diagnose it whilst it is missing?
Cheers
Mark

junkcatcher · #6 Post by **junkcatcher** » Wed Sep 23, 2009 6:00 pm

Symptoms do fit a plug or HT misfire

Specified plug gap is 1.1 mm (0.044" in old money) before doing anything else try resetting the gap to 0.8mm (0.032") and road test.

Doen't sound like a burnt valve --- as this usually causes a very rough idle and lack of pulling power in the low to mid speed range accompanied by very heavy fuel consumption. Much the same symptoms show up for a head gasket leaking between adjacent cylinders.

Sticking valves, a broken valve spring or hydraulic tappets are more likely --- sticky valves can sometimes be freed by dribbling Redex into the throttlebody intake -- however it makes massive clouds of anti-social white smoke. Sticking hydraulic tappets can sometimes be freed by using Wynns Hydraulic lifter treatment or adding 500ml of Dextron 2D transmision fluid to the sump as a flushing/ungumming agent and running for a few hours before changing the oil.

billynoband · #7 Post by **billynoband** » Wed Sep 23, 2009 6:01 pm

Have you tried the Jeep forum? Good luck.

BigM · #8 Post by **BigM** » Thu Sep 24, 2009 10:16 am

I can't remember what gap I used off the top of my head, but maybe I'll check them again. I know that I did set them to the specified gap and it misfire does happen on either fuel.

A bit more description.... If I floor it from standing start, or floor it at say 30mph, the car pulls fantastic without any misfire at all, all the way up to 5500rpm and above (at least I can't notice any). It seems to only happen when at motorway speed (70 and above).
If I'm holding at a steady 80mph you can feel a strange "Blip" about once a second. If I put my foot down a bit whilst at 80 it still pulls ok but not very smooth at all (as if it's misfiring).

I have posted this on the Jeep forum also....waiting to see if I get any responses.
Thanks to all with suggestions so far.

junkcatcher · #9 Post by **junkcatcher** » Thu Sep 24, 2009 11:57 am

From your last post I am starting to think iIt might be an idea to check the upstream Lambda sensors output and the throttle position senors out puts.

Lambda sensor only really plays a major part under steady state conditions but your problem appears on transition from steady state to acceleration. --- lambda sensor rarely raise a fault code unless totally goosed so it is really a question of looking at the voltage outrput either directly or via live OBD data. The lambda sensors output should

Throttle position sensors work much the same as a volume control on an old stylr TV or Radio , because most driving is done with light throttle they tend to wear out first at the start of the throttle movement. Like a worn volume control a faulty throttle position senor will give a sudden jump up in output voltage which can confuse the engine ECU.
Best way to to check a throttle position sensor is with an oscilloscope to look for glitches in the output voltage is the throttle is moved but voltmeter can also be used as can live OBD data.

The other main sensor it might be is the coolant temperature sensor, this has a major effect on fuel mixture on both fuels. Easy enough to check and I suspect the Jeep dealer may have carried this out already.

BigM · #10 Post by **BigM** » Thu Sep 24, 2009 12:14 pm

Thanks Junckcatcher for your suggestions.

Did you get half way through your sentence about the lambda sensor? "The lambda sensors output should......."?
Not sure whether it would be the throttle sensor as I get a small "blip" misfire thing about once a second when holding it steady at 80mph.
Is the temp. sensor you're refering to completely independent from the gauge on the dash as that reads fine? Also my fuel consumption is fine.

The jeep diagnostic thing that was plugged in to check faults was at an independent specialist and not a main dealer, although the unit did have Crysler Jeep written on it. They said that if the car had misfired at all in the past it would be recorded and show up on this thing......nothing shows up on it tho.....would the dealer have access to better diagnostics?

Cheers to all

alexf2828 · #11 Post by **alexf2828** » Fri Sep 25, 2009 10:43 pm

Maybe firing order wrong- HT leads on the right plugs? has the installer got the injector firing order right? has it always misfired since the install? also try gapping plugs to a smaller gap and try wiring a remote spark plug to each HT lead in turn to see if spark is there-maybe the output from the ECU has gone down and no pulse to the coil

gavbriggs · #12 Post by **gavbriggs** » Tue Sep 29, 2009 5:02 pm

i had a problem with the ht leads breaking down under load on a escort, similar sort of symptoms.

is there a crank position sensor on the cherokee?? i know on range rovers they can cause all manner of missfires if the contacts are out of line/bent or the sensor is dead!