dylansmith17

1995 pickup V6 3.0 4x4, manual, 103k miles

I've had this truck almost two years and ever since I can remember it's had a slightly noticeable misfire/rough idle/vibration ONLY at idle. Acceleration, power, and MPG are fine (for a 3.0) and I can't feel anything miss once I start moving. I had to smog it last week (CA resident) and it failed due to high hydrocarbons (unburnt fuel), which the tech said is analogous to running too rich. Did a compression test and I have 180-190 psi in all cylinders. I replaced front O2 sensor (there was a ˟˟˟˟ty Bosch one on there before, replaced with the correct Denso), plugs, wires, distributor cap and rotor, took it back, and it passed with flying colors.

BUT after I replaced all that my idle is now way lower, right around 500rpm, and the rough idle is VERY pronounced. At idle it feels like it's constantly about to die, but again everything feels fine once I start moving. Even before the smog thing came up, I've also have an intermittent CEL that comes on ONLY when I'm flooring it up a hill and downshift from 5th to 4th, i.e. when the engine is working really hard. Then I get to the top of the hill, and it turns itself off within 30-50 miles or if I manually resent the ECU. Well today I finally got around to jumping the diagnostic port under the hood and see it's a code 27: CA emissions-specific code related do the downstream (after the cat) O2 sensor. I looked under there and the wiring looks good, but no idea when it was last replaced. It's the right Denso one too. Could be the original.

But before I go replace it, could this code be related to my rough and low idle? Anyone with a CA vehicle ever get this code before? I did a search but most guys who've posted about code 27 immediately find a broken wire or something. Would be psyched if I could fix this 2 year old idle problem with a $50 O2 sensor and 1 minute with a 10mm wrench!

Thommo Thompson

i going to ignore ur code 27 and focus on the factbthat u replaced a bunch of stuff and got a lower idle.
to me, it sounds like the old o2 had failed and the car ran very rich as a result, iether due to running in open loop mode, ir getting a genuinely wrong signal from the O2 sensor.
oxygen leaks in the exhaust can gibe these codes, but the fact that uve had a change in idle speed makes me think the main O2 sensor was indeed no good.
so now u havr a good O2 reading the car is running leaner, which will make any vacuum leak, fuel preasure issue, or spark quality issue more noticable.
the fact that u are only noticing the missfires on idle suggests there is a relitively small vacuum leak. small vacuum leaks tend to be only noticed at idle, when vacuum is high, and airflow is minimal. before u had an over rich mixture so that looked after it. now u have a correct mixture but the leak is causing it to lean out at idle. check everywhere for vacuum leaks. spray water o to every hose and hose connection from a spray bottle and listen for a drop in rpm.

dylansmith17

Thanks Thommo. I will check for vacuum leaks when I get a chance in the next few days.

Would someone care to illuminate a noob. I understand that vacuum is highest at idle. I understand that vacuum controls many different systems in a car, e.g. brake booster, EGR valve (on my engine at least), etc. But many of these surely are not related to a misfire at idle. How could a vacuum leak anywhere in the system of many vacuum lines connected to the motor cause a misfire and rough idle?

And for the code 27... anyone who has had one of these pop up solve it by just replacing the downstream O2, or is it usually caused by something else?

Thommo Thompson

not a noob question, that much is actually quite technical.
the cars computer measures the amount of air coming in through the throttle body and gives it the appropriate amount of fuel according to that measurement. at idle, a small leak is a large amount of unmetered air, as the throttle plate is effectively closed and the only air that doea get in is through a very small idle air controll valve. at idle, the computer is particularly sensitive to a leak.
and u very cleverly have workes out that this basically doeant apply during on power or high rpm situations, where the amount of air already going into the engine is allready extremely large and dwarfs any phantom air from a vacuum leak.

so, in short, its not that the vacuum is required for function, its just that the term vacuum leak is used to describe any unmetered airflow leak. perhaps it shouldnt be called a "vacuum" leak.

millball

The downstream O2 sensor cannot cause your issue. Its only function is to monitor the efficiency of the CAT, it does NOT contribute to engine management.

dylansmith17

Update: I checked for air leaks and as far as I can tell, I have none. I sprayed carb cleaner slowly over all vacuum lines that I could see while the engine was cold and in it's warm up cycle. Noticed absolutely no deviations in idle speed. I particularly focused on the area where the throttle body mates to the plenum, heard when that leaks it can really ˟˟˟˟ up your idle. But got absolutely nothing. I guess maybe I'll try to diagnose my EGR valve next to see if it's opening and closing at the right times - anything else anyone can think of? I don't have a lot of experience driving other 3.0's (I've driven my buddy's, once) but as far as I can tell the thing has plenty of power and the misfire is at idle only. The thing shakes sooooooo much at idle.

The only other thing I notice is that the valves seem REALLY loud at idle, especially when the truck is warming up. Again this is the only car I've ever owned so I don't have much to compare it to, but at least I can say I've never noticed valves being that loud on any of my motorcycles. It's like a loud fast tapping/clicking sound, almost like a buzzing, from the top end. Guess I don't know but I'm assuming it's valves. Is that normal for these engines? And again if the valves were that out of spec wouldn't I notice a lack of power? Thanks all.

Thommo Thompson

ok go back and try again with soapy water in a spray bottle and this time listen for a decrease in rpm.
U must do this, because sometimes solvents just simply burn (effectively a little richer) and make no difference to rpm.
If this then also comes back negative, then perhaps start looking a the cause for the loud valvetrain.
There is no adjustment on these motors because the valves are pushed down by bucket valve lifter with a shim over it...
https://f01.justanswer.com/ref/http_...5280060a23.gif
That shim can wear out, but also, I just finished dealing with someone who actually had one of these shims completely jump out of the bucket, so have a look and make sure they are all in tact and in good shape.
If they are worn out completely, they will make noise, and can affect the idle more than the power, so it may even be your entire problem.

dylansmith17

OK, will try that. Am I right to do this when engine is cold and warming up? I read that several places but it seems odd to me, as the engine is revving higher than normal idle anyway during its warm up cycle, but hey what do I know.

And really, there is no valve adjustment for a 3.0? I swear I've read of multiple people adjusting valves on the 3.0. That being said it sounds like a huge PITA. I appreciate your advice and comments but I don't think I'll even get as far taking the cover off and checking clearances. I've checked and adjusted valves both with and without shims on other engines but the procedure on the 3.0 sounds above my skill level. And I won't be paying anyone else to, as I'm trying to stick to the mantra that everybody else swears by: don't spend any money on the 3.0. Save your money for a 3.4.

Thommo Thompson

ur more likely to notice a vacuum leak in an efi car once warmed up. so the test, if being performed on an efi engine is best performed when warmed up. this is because the fuel mixuture is leaned out in closed loop mode (mode ecu goes into when hot)and phantom air leaks have more an effect in this mode.
so perform the soapy water test with engine hot, and just a light spray around hoses and connections and gaskets should be enough to get through any leak. and yes, a high idle is the worst time to check for vacuum, because the throttle plate is more open and an air leak is less relevant, even when saturated with soapy water. so nice hot low rpms.
as for valve adjustment..

so the valves have the spring then on top of that is the bucket and in that is the shims. these shims can be replaced with different sizes but this shouldnt really need to be done unless u have had camshaft work done or the engine is incredibly worn. there is no screw adjustment per say. so yeh, bugger that leave if.
i could learn something from that mantra, ive just spent a whole week getting a clapped out old completely wotn toyota 4k motor in running condition. everything, and i mean, everything, gave me troublea during the rebuild....
4 stripped female threada in aluminium head, which required bigger bolts and tgerefor serious machining to other parts, bearings were destroyed and upon reinstallation found that previous engine builder had installed caps backwards which ruined the crankshaft...
all coolant passages in head corroded beyond the seals of the headgasket, repair involved epoxy resin haha.
pistons so loose they can be wobbled left and right by hand in their bores abour 1 mm....
no gaskets available for exhaust....
no pistons available...
no cylinder heads available...

in short, your rule of not wasting money on it is a sensible one in my opinion.