Greetings all,

Many of the topics and DIY Articles have been very helpful and I thank those posters for their efforts.

I have a '01 Jimmy @ 155,000km. Recently a "miss" was developing at idle only, typically after engine has warmed up. A hesitation when accelerating from stop was also developing.

Considering the mileage, decided it was time to replace Plugs, Rotor, Cap, Wires, Fuel Filter, Cleaned MAF and EGR Valve. Each of these items were changed sequentially, with a few days of drive time between each item. AC Delco for all parts except the wires were a reputable quality after market recommended by the AC Delco parts house.

Incremental improvements on driveability were noticed after each component was changed (ie. responsiveness, shifting was more predictable, etc.).....overall good improvements.

But none of these parts has eliminated the random misfire. It still occurs only at idle as it was prior to the tuneup parts. The hesitation from standstill is much less frequent but happens on occasion.

I don't have a scanner, but a shop hooked up for me and showed/explained the display and codes while the engine was operating through various RPM, etc.

The random misfire occurs exclusively on cylinder 3 and only when at idle. As soon as any throttle is applied (even to just 700 RPM), the random misfire count on that cylinder goes to 0. Allow it to return to idle and the misfire count begins racking up....just on cylinder 3.

At each occurrence of the random misfire, a distinct "tick" can be heard from the driver's side of the engine (where cylinder 3 is located). It can't be heard from the opposite side.

The engine has never stalled due to the misfire, it starts promptly without problem and it seems to operate at driving speeds without concern. It's just the misfire at idle that is frustrating.

Fuel pressure checked out at 55psi promptly after turning key ON. Maintained +55psi during engine run and retained 55psi for a couple minutes after shut off. Didn't suspect problems here, so disconnected.

Our suspicion is a marginally sticking poppet causing the misfire and corresponding "tick". But not sure why it completely disappears at any RPM above idle.

Any other comments or suggestions on trouble-shooting further? i.e. is there still a possibility of an "electrical" related misfire caused by the coil or ICM?

Thanks!!

-Jeffr

 

It likely is a poppet issue. It likely is not sticking, but just isn't spraying properly. At anything off idle, the speed of the engine and the higher fuel demand masks the issue.

I would try a stiff dose of Chevron Techron Fuel System Treatment Plus (two 20gal bottles at once right before you fill up) and see how that affects your condition.

 

Thanks swartlkk,

I'm in Canada, so I don't have access to the Chevron product. But the shop provided a can of Forte Injection Cleaner. We're running that through the system now and will see if that helps out.

Are the poppets known to make a "tick" sound when they are misbehaving? Or could we be looking at a sticky lifter or valve?

We had noticed during scanning that the " CPM...advance" (sorry can't recall the exact term) was running at -6% (instead of commonly 0 to a positive %). The tech at the shop I go to indicated that GM would suggest that the timing chain should be replaced (but he's not convinced of it). Apparently this is a result of not being able to adjust the distributor, as is typical on the older style where you rotate the distributor to adjust timing. However, sometimes enough slack is available in the distributor mounting location holes (or can be slotted a bit), to override this issue.

At this stage, we're not too worried about this issue, but I thought I would mention in case it may be relevant for ideas on the idle miss.

Appreciate the assistance!

-Jeffr

 

Timing would affect all cylinders, not just #3. The injectors/poppets tick as they operate so that is normal.

I am not sure what you are referring to as CPM... That doesn't sounds like one of the parameters that my scantool reports...

As far as fuel system cleaners go, GM has their branded cleaner that will do the job as well.

How do the plugs look, comparing the #3 plug with the others?

 

Yes, the effect of the timing chain on all cylinders discounted that issue. As well, we eliminated Fuel Regulator, probably coil and likely ICM for the same reasons....it must be something that can uniquely affect cyl 3.

Mostly discounted Distributor Housing/Shaft as there was no Lateral play when changing rotor, but there was some noticeable axial play (vertically) maybe 1/8"....I suspect this is normal.

The value I referred to was with regard to overall timing advance that the computer was adding or removing (in this case removing 6% at idle, I believe).

I'll check around for some other quality cleaners (Seafoam perhaps), but since we've started with the Forte one, I might want to stick with it for a couple rounds.

When changing the plugs originally a couple weeks back when the problem was developing, none of them looked particularly bad compared to each other....I still have them. Labeled them, too just in case.

I'm going to pull a couple again later today and review.

-Jeffr

 

The computer changes timing based no a number of different inputs. Timing at idle should be advanced pretty far (20-32 degrees BTDC), but will bounce around depending on the conditions. Your initial statement said that this CPM reading was at -6% which does not make sense for a timing readout which would give degrees before TDC.

 

I'm not certain what the reference point is for the -6 reading (maybe it was degrees and not %...going from memory here), but the tech was familiar with the values and wasn't particularly concerned when considering how the engine operates otherwise.

Perhaps it is removing 6 degrees from the initial timing value of 20-32 BTDC. I don't think it was the actual degrees of timing value, but the amount of adjustment being applied by the computer.

The tech had just commented that GM would typically recommend timing chain replacement for this situation, but because the engine runs well otherwise he's dismissing it as a cause for the misfire. Because I can't recall the specifics very well, I'm not questioning his diagnosis. He has been very, very helpful and forthright with his assessment.

The shop and their tech/foreman that is helping me have been tremendous and considerate even though I'm doing much of the work myself. I'm inviting suggestions and opinions as we work through the problem and hopefully our diagnosis will be of assistance to others on the forum. Trying to give back something for what I've gained here (Balljoints DIY, Front Hubs DIY, Pinion Bearing @ Rear Differential DIY, Blower Motor DIY, etc., etc. sadly the list goes on....but so it seems to be with this vehicle).

Once again, thanks for the help!

-Jeffr

 

I am agreeing with the tech's advice. I do not feel that timing is your issue.

 

It certainly seems that the problem has been narrowed down to a misbehaving injector/poppet.

The Forte product is improving the situation significantly. There is still a slight "tick" sound on occasion, but the misfire frequency and harshness has dropped off dramatically.

This is what we used: http://www.fortesystems.ca/pPrducts/...%20Cleaner.asp

-Jeffr