Very Rough Running when cold - Mechanic and Auto electrician can't diagnose

[graham7773]

Would the crack in the manifold allow air/oxygen into the exhaust system giving a high oxygen reading through the oxygen sensor on that side? Would that affect the amount of fuel the injectors put in? ie too much?

 

[acarmody]

Would the crack in the manifold allow air/oxygen into the exhaust system giving a high oxygen reading through the oxygen sensor on that side? Would that affect the amount of fuel the injectors put in? ie too much?

Not a hairline crack. Also the amount of pressure in the exhaust probably wouldn't allow into it, only out.

 

[wayne70]

98 vt

hi guys i have a 98 vt 3.8 auto and its running like **** its got a bad miss when you put your foot down iv replaced the plugs leads and coil cluster pack and its still running like **** any idears would be great thwnks

 

[db_notso]

I like the idea of unplugging the coolant temp sensor...does that fool the car into thinking it is running warm?

One other thing... the mechanic said there is a hairline crack in the exhaust manifold - although he also said that this should not be causing the rough idle when cold issue. What do you think?

a hairline crack will alter the engines performance on one side only
when hot it would cause a state of overfueling as not all burnt fuel is reaching your o2 sensor
when cold it will in a minor way affect the air fuel ratio by imbalance in tune on each bank
as it all runs on a predifined settings until warm (very basicly explanation here)

yes unplugging it while running sets a code and the ecm defaults to warm setting fans on full

suggest getting a wreckers manifod and fitting it to rule that out as a cause

that is good of them~ they sound like alright blokes then...
for interests sake i would like to know what "pointless" codes were in there
but dont push them or it will annoy them resulting in your pain...

 

[wrx884]

try doin the self test 4 the codes that db_notso said and report back.

 

[vsd]

An update guys

The mechanic did the pressure tests and also tested for moisture/coolant in the cylinder and found no issue.

However after putting the plugs and leads etc back on he can't get the car started at all now. He said it is trying to start but doesn't. He said it feels like the same issue (as initially described at top of this posting) but just a whole lot worse. The mechanic has towed the car back down to the auto-sparky to see what he can find.

What do you guys think? It makes me think it is something maybe related to what he removed/changed in order to do the pressure test that has made the issue worse?? The plugs, leads and coil packs have all been renewed, what else could there be?

(I will try to get the fault codes from the auto-sparky.)

 

[Wherry951]

Well i fixed my problem, of a similar nature to yours. Turned out i had a big split in the air intake pipe, so replaced that, as well as a dirty air filter and air sensor. The air sensor was dirty so it wqasnt getting a true reading, so it let too much fuel in, causing the car to choke and not run properly And this also was the cause to my V6 getting only 420kms to a tank! Took 20 minutes, now it pulls away like a rocket, uses no fuel, runs fine. So i'd be checking that as well as checking you dont have Bosch spark plugs, i made that mistake, only got 1000kms out of them

 

[lout]

I would be fitting another crank angle sensor as symptoms started after that was replaced
try when engine is extrememly cold running a heat gun (hairdryer) on crank angle sensor before starting to see how it starts when crank angle sensor is warm

 

[vsd]

Thanks wherry951 and lout, those ideas sound good I will let you know how things go. Awesome thanks!

 

[wrx884]

pouring hot water on the CAS while ur trying 2 start the car will tell u if its the CAS or not. if it starts its the CAS if not its somthing else.

u dont need the sparky 2 get u the codes man try doing it urself as sent u b4.