3.8l VT hesitation / misfire under light load when warm

[Reuben Campbell]

Hi Folks

Car is VT 2001 Executive wagon 3.8l , 340km - and showing every km on it - its hard a hard life I think before it came to me in 2016.
The engine is very tired, but performs just fine for its intended purpose.
The car is just a hack , but it still deserves some basic maintenance. It wasn't supposed to last thins long, but it wont die.

For the last 12 months, its been having this half misfire / hesitation thing going on when the car is under light acceleration, in 4th, with light load, like going up a hill.
It seems to only occur once its at operating temp, and gets worse in hot weather. Dropping it into 3rd would fix it.
Recently, it seems to be doing its hesitation trick now when in 3rd and medium acceleration.

Its also having trouble idling smoothly. It will sit fine for about 20-30 seconds, then just die and pick itself back up again without stalling.
I'm guessing both are related somewhere in the ignition system or a sensor.

From my reading i thought it was a coil or the plate under the coils (DFI unit?). I found some youtube videos on testing them, and they all tested within spec.
None the less, I swapped out the DFI unit and coils from a spare I had in the shed - no change.
Now the the coils and DFI came from a VXII so they could be just as buggered.

Before I start on just replacing every part on the engine, is there a good order i should start on?
is the ebay stuff any good? It all seems pretty cheap, so its in the right price range for the purpose of the car, but there is no point if they are unreliable or just going to fail in 6 months.

I'm thinking
coils
DFi plate
plug leads
CAS sensor (the rough idle?)
to start with.

Does anyone else have a better plan / opinion?

Thanks in advance

 

[accentstencil]

I'm going with leads. Whenever I've had a miss under load over the years, many, many years it's always been a bad lead.
It could also be a plug though. If you haven't changed the leads in years it wouldn't hurt to do so.

 

[Reuben Campbell]

I'm going with leads. Whenever I've had a miss under load over the years, many, many years it's always been a bad lead.
It could also be a plug though. If you haven't changed the leads in years it wouldn't hurt to do so.

Fair call - Ill get a set of leads to start with - its a good a point as any.

Does brand really matter?

I just saw these
https://www.ebay.com.au/itm/1259819...kEgxk8gpcucasvFjdFRk|ampid:PL_CLK|clp:4429486

and they are red - and you know what red parts do... they go faster!

aside from that - that seems to be the going rate for leads ($40) + coils ($35).

Plugs were done at last service - but next service is due in 1500km, so its about time they get changed anyways

 

[accentstencil]

I don't know about those leads but always like to get a well known brand.

 

[Reuben Campbell]

So, Last weekend, i went and purchased a set of ignition leads from my local auto store.
I was surprised at the cost, ($120) but I wanted to resolve the issue, so hand over the folding, and off I go.

After some cursing that I have to remove the alternator to the get the leads out and back in....

i had success!! the intermittent misfire seems to have gone! I haven't had much chance to do some solid testing, but so far so good.

Thanks for the hot tip on the leads.

Now, I just have to sort out why the central locking has failed. I think its the BCM that has failed.


The rest of the cars problems (there are a few), ill get to eventually.

 

[Reuben Campbell]

So, Last weekend, i went and purchased a set of ignition leads from my local auto store.
I was surprised at the cost, ($120) but I wanted to resolve the issue, so hand over the folding, and off I go.

After some cursing that I have to remove the alternator to the get the leads out and back in....

i had success!! the intermittent misfire seems to have gone! I haven't had much chance to do some solid testing, but so far so good.

Thanks for the hot tip on the leads.

Now, I just have to sort out why the central locking has failed. I think its the BCM that has failed.


The rest of the cars problems (there are a few), ill get to eventually.

Well it seems that my success was short lived - bugger.

Its been back to its usual games now for the last 2 weeks. I'm only now got time to get to it.

When I bring the engine idle up slowly, its runs fine all the way to redline.
If I stab the throttle, its starts its missing caper, running rough.
When driving, as long as I am at about 5% throttle, it runs fine, but anything above this its starts to miss and carry on, running on about 4 cylinders.

The idle also struggles to keep constant, even without aircon on.
It will idle fine, then about every 2 minutes, the idle drops down about 150-200 revs, and it picks itself up again and then idles fine.

So far;
I've since swapped the coil packs and DFI plate with a spare second hand one. - No real change.
just serviced - new plugs, air filter, oil, oil filter.
I've plugged in my Envious Customs AED plug - no errors displayed/picked up , and no dash light errors.
using MAF sensor cleaner, cleaned the MAF sensor in the airbox pipe.

before I run down to the auto store and buy a DFI plate, coils and a CAS, I'm starting to think its a sensor elsewhere.

Would the Throttle position sensor or the Idle air sensor cause all this?


Other known issues - in case they matter
leaking exhaust at the 3 into 1 join on the drivers side. exhaust is 2" and very open. Not flash, but it was when it was fitted well before my time.
central locking remote doesn't work - key in door works fine, just not the remote.
power steering factory feature leak
suspension all round is floppy/soggy/sagged, but it keeps the body off the wheels...
uses about a cup of coolant every week or so - not into the oil or leaking that I can see - goes somewhere.

 

[Lex]

Well it seems that my success was short lived - bugger.

Its been back to its usual games now for the last 2 weeks. I'm only now got time to get to it.

When I bring the engine idle up slowly, its runs fine all the way to redline.
If I stab the throttle, its starts its missing caper, running rough.
When driving, as long as I am at about 5% throttle, it runs fine, but anything above this its starts to miss and carry on, running on about 4 cylinders.

The idle also struggles to keep constant, even without aircon on.
It will idle fine, then about every 2 minutes, the idle drops down about 150-200 revs, and it picks itself up again and then idles fine.

So far;
I've since swapped the coil packs and DFI plate with a spare second hand one. - No real change.
just serviced - new plugs, air filter, oil, oil filter.
I've plugged in my Envious Customs AED plug - no errors displayed/picked up , and no dash light errors.
using MAF sensor cleaner, cleaned the MAF sensor in the airbox pipe.

before I run down to the auto store and buy a DFI plate, coils and a CAS, I'm starting to think its a sensor elsewhere.

Would the Throttle position sensor or the Idle air sensor cause all this?


Other known issues - in case they matter
leaking exhaust at the 3 into 1 join on the drivers side. exhaust is 2" and very open. Not flash, but it was when it was fitted well before my time.
central locking remote doesn't work - key in door works fine, just not the remote.
power steering factory feature leak
suspension all round is floppy/soggy/sagged, but it keeps the body off the wheels...
uses about a cup of coolant every week or so - not into the oil or leaking that I can see - goes somewhere.

Just buy dfi from the wreckers. Have 2. One for a spare.
Replace the exhaust manifold.
It sounds to me there maybe a inlet manifold gasket problem. Whats under the radiator cap & oil cap?
Are the cas & cam sensor plugs clean?
Also whats the dfi wiring plug like? no bent or burnt pins Have you stripped back the tape & plastic on the dfi wiring plug?
Do you have the correct iac in?
How old are the spark plugs?
Where has all these parts come from?

 

[Reuben Campbell]

Just buy dfi from the wreckers. Have 2. One for a spare.
Replace the exhaust manifold.
It sounds to me there maybe a inlet manifold gasket problem. Whats under the radiator cap & oil cap?
Are the cas & cam sensor plugs clean?
Also whats the dfi wiring plug like? no bent or burnt pins Have you stripped back the tape & plastic on the dfi wiring plug?
Do you have the correct iac in?
How old are the spark plugs?
Where has all these parts come from?

Thank Lex
I have 2 spare DFi units here in the shed - none of them have improved the outcome. Repco have a special on these at the moment (https://www.repco.com.au/parts-serv...es/fuelmiser-ignition-module-cm414/p/A8784387)

Have just had a look under the car again and have fixed the manifold leak. there are some more leaks in the pipes further back, but that's not the problem now.

radiator and oil cap situation. both are fine. No water in the oil (just changed the oil today). no sign of oil ingress into the radiator or on the cap.

CAS and CAM sensors -
CAM sensor (the one just above the bottom pulley) - cleaned and no change. some minor carbon build up.
CAS sensor (the one behind the bottom pulley) - not yet removed. If this is likley culprit, Ill have to buy the tool and I may as well replace that then.

DFI plug looks fine. have pushed electrical cleaner through it. no bent pins that I can see. Plug looks clean. No burnt wiring visible. Wires bend with no insulation falling off or cracking.
Have removed the insulation tape back to the next junction (about 15 cm) and all fine.

correct IAC? its the one thats been in the car since I bought it in 2016. removed and cleaned this also. some carbon buildup here, but no change after cleaning.

New 8mm leads 2 weeks ago, new plugs two weeks ago (NGK), oil and oil filer, air and fuel filter changed today.

A friend just told me that he noticed that it is running super rich, so much so that they can smell fuel as I was driving away.

all parts from auto one in Midland, Perth, WA.

 

[Lex]

A friend just told me that he noticed that it is running super rich, so much so that they can smell fuel as I was driving away.

Check the fpr = fuel pressure regulator.
If fpr is ok, check the o2 sensors.

 

[Reuben Campbell]

Check the fpr = fuel pressure regulator.
If fpr is ok, check the o2 sensors.

I've done what I can with the FPR. there doesn't appear fuel leaking into the vacuum hose so I think that the diaphragm is fine (ie not split).

Its strange. It idles fine ( aside from the random drop in revs every so often), it revs up fine if you do it slowly, but give the pedal a stab, and it is rough as buggery.

with the induction noise, its hard to hear if there are any other vacuum leaks, but i did spy some coolant in he valley at the back. i guess thats where my coolant is going. Must be a coolant line there somewhere that is weeping.