LS - misfire

Just got a 2001 100k ls v8 for $1700... someone disabled the "check" engine light but I have a reader.

Misfire on #2 and #5, only seems to miss on high revs in gear on the road.. So I ordered 8 plugs and 2 coils. ( so I put it in drive applied break, gave some gas, don't have any missing, it seems smooth). Go floor it on the highway around 4k-5k you can feel the miss its in and out. This car sat awhile I think and thought it was just bad gas. After running through the "old" gas, a lot of the miss on the highway floored was "clearing up"

It starts right up, runs well no noise in the engine....

I would assume this is the first step, replace to coils & plugs

I don't think its missing all the time..

Besides the injectors, would it be safe to assume there is no fuel pump issues?

thanks

Check for codes again after putting in the new coils and plugs to see if there are still codes for cylinder 2 and 5 misfires. While you're waiting for the plugs and coils you could check fuel pressure to see if the pressure is within spec. Although if the issue only happens at high engine speeds, it may be a bit tough to get a good fuel pressure reading at high RPMs. There is a rev limiter around 3500 RPMs in Park/Neutral.

-Rod

I will do.. starting the change tonight..



Hope the CAT is ok and its not dumping to much gas into the exhaust! ( does not smell rich...)

thanks again...

I switched out the coils #2 was some aftermarket coil green sticker looked new. Once changed #2 does not come up as misfire..

#5, no so lucky no change. That coil was changed too.. pulled plug it looked new and awfully clean ( the electrode to this plug is a tiny point gapped ok) , lil tiny carbon around the bottom. It was not wet, very dry.

The valve cover on the drivers size has let go on about 2 bolts for the coil cover. So that was missing.

Noticed some "rust" looking color on the coil wire on #5 as if it was stained with rusty water. The connector looks fine, although i broke the locking clip. i made sure it was on all the way during tests however. The injector wire clip i broke. still could not get off the injector wire to make sure its ok i am assuming its ok..moving the wires around i got no change in running so i dont think its an injector.

Another interesting observation, I can feel the miss now at idle. and it throws the p035 code for missing #5. I cleared the code turned off the car. Then i reconnected to make sure no codes were thrown. I then started the car but held it at around 2000 rpm where the miss seems to clear out. The code was not thrown i ran it at 2k for at least a min ( at idle the code comes up right away). So holding at 2k it did not throw the code once it goes back to idle code is thrown. maybe this is too fast or takes longer for the system to decided if its missing.

In addition when you first start it you cannot feel that miss as that the car is in a "higher" rpm when you first start. As the rpms come down to idle, you can feel that miss come in. SO it almost is like the injector has a "idle" mode in which maybe thats the miss. ( i keep saying this is just like a bad injector on my mustang i had, but missed thru the entire rpm range..)

I think the first thing to do is figure out if i am getting signal into the injector and the coil. Can i put a meter on these wires to check for signal?

Lastly you put it in gear, you feel the miss slightly.. You give is some gas like to about 1500 rmp 2000 rpm. you dont feel that miss.( may this engine is really balanced and its there just dont feel it)

overall it seems its an injector.. but maybe someone out there has a better idea ...

thanks so much!

You can check for signal to the injector and coil primary, but you really need an oscilloscope for either of those measurements. A typically multimeter is not fast enough to accurately see the pulse and determine if it is doing what it should.

An interesting experiment could be to unplug the #5 injector and see if the engine runs any differently at idle and at higher RPMs. You could try the same with the #5 coil, then pull the plug and check for wetness.

-Rod

Hi - thanks for the info, I will perform the test you suggest today and see what happens.

i have an old oscilloscope here not sure it works.. ill see if it powers up. ( its an old ham operators scope. really old like from the 1960's!)

thanks

update

I was up late last night searchin on google for every miss on a ls. Before starting new tests today the new plugs arrived. I changed out the plug on 5 only. This time the plug i pulled out looks like it was firing normal ; light tan so maybe it was firing now after the coil change. The car was drivin about 20miles since working on it yesterday.

The new plugs look somewhat different. The "ngk" plug which was in it has a point in the electrode, like a small wire.. This new plug, has the point but its designed like a round pyramid, bigger at the base and goes to a point. its gapped ok out of the box..

but none the less i cleared the codes, miss came back on the computer. although took a bit longer

While searching i found that some say the ls computer needs to be "reset" after correcting the hardware for any "miss" issue. So thats what i did. I removed the power from the car for a bit (bout 5mins), reconnected and fired up.

5 min went by no pending miss on 5
10 min went by no pending miss on 5 o
15 min went by no pending miss on 5
shut down car replaced coil cover retightend the batter postive, went around the block.

still no miss on #5. HOWEVER I still feel alittle stumble at idle

Its a really wet damp cold day. if there is anyday for a engine to miss its today.

i will post update after driving out more... not sure if this is now a reset of the computer which was needed and the coil and the plug, or the code just has not come up yet.

( i hear the injector clicking via a screwdriver so i can cross off no signal to the injector)
thanks

You really shouldn't even attempt to check the gap on most newer spark plugs. The electrode is typically either platinum or iridium and checking the gap can damage the electrode.

Since you cleared codes and reset the PCM, you should give the car at least a complete drive cycle to stabilize before getting too concerned about a slight stumble. Resetting the PCM blows aware all "learned" parameters so it might take several miles to smooth out.

-Rod

update - everything seems OK. The car is being drivin around, and I keep checking the computer. All tests have passed (catalyst test has passed!), waiting for the "evaporator" test to complete. no miss codes. Since the coil change then the "rechecking" of the plug in which it now looked "working",after the cop change; this must have been the coil all along I think. Resetting the computer did that "relearn" which cleared the persistent miss code in 5, but really was not missing anymore, but had the fuel cut off. ( it seems) (The miss in #2 went away as soon as I changed no computer reset was needed for the #2 cop and replacement.)

It does seem to vibrate the car lil at 650rpm idle as if it had a bumpy cam, as that at 1000 rpm its smooths it out perfectly. The exhaust must flow well as that there is not "float" when revving this engine... There is a slight exhaust tick it clears up when warm. The transmission has gone from very firm shift points to really smooth shifting now.. Going to get transflush sooon.

I once again "probed" areas with a long screw driver and listened through it to detect any internal ticks in regard to the timing chain problem. The engine sound smooth.

The cop "plug" on #5 had a broken locking clip. I put a large plastic wiretie around the whole cop and wire, it pull the clip under the wire. it should hold ok.

I think this engine has headers not exhaust manifolds. The headers look brand new, like off the shelf. I should take a pic, for a 105k mileage car.

This car has full working posi rear! My mustang didn't have that.

Going to replace the air box with a adapter and k&n filter. Seems to be the "intake" is a pretty small intake tube into the box.

thanks for the help