Blazer's engine stumbling/surging, but no codes...

So, I've finally got pretty much all of the issues worked out with the knock-knock joke Blazer save one. When idling at a light, it will either noticeably miss/stumble- enough to feel it and hear it if you're listening- or idle nicely but every 5-6 seconds or so surge up about 500 RPM and back down again. Which it does seems to depend on the temperature- when it's cold out (under 40F or so) it seems to do the stumbling thing regardless of engine temp, when it's warmer it surges.

It also tends to stumble/hesitate when at very light throttle after it has just shifted up into 3rd or OD- a quick tap on the gas to bump it back down to the lower gear. It has not troubles otherwise when giving it a modest to good bit of gas, and pulls just fine when you get on it. It was for a while throwing a code for the idle speed, but a new IACV resolved that and it idling about 500 RPM higher than is should have been.

As near as I can tell, it's not a vacuum leak problem- I've hunted around and not found any signs of that. The cap, plugs, and wires are all a few hundred miles old. I swapped out the TPS that was on it for one from another junkyard throttle body and it didn't make any difference- but at the same time I had done the same with the IACV and not seen a difference but the brand new one made a major difference.

Any suggestions as to where I should be looking/what I should be doing next?

this is a 96+ vortec right? have access to a scan tool or a bluetooth plug and a scanner program?

i could not take it anymore with mine. i had a similar issue, had great fuel pressure, new filter, switched the entire intake with injectors, sensors, distributor, coil, and throttle body over from a truck that ran perfect, and could not get rid of the miss. the vortec 4.3 are a mystery to me.

I seem to remember my buddy a long time ago having an issue with a 4.3 that turned out to be a wobbling distributor shaft. Caused all sorts of weird intermittent issues. I never saw the truck in person but it seems to make sense.

Yup, it's the 4.3L Vortec. I've got a decent scan tool, but am not seeing anything in the way of codes or know what beyond that I should be looking for. I've seen a few things in searching that hinted that worn distributors could be a problem- and that (the distributor itself) is one of the things that I did NOT replace when I swapped in the new engine, that might be the next thing I need to try.

If you can see data on your reader, look for cam offset. It should be as close as possible to zero, anything more than 7-8 degrees will cause crossfire issues in certain conditions. Its adjusted as if you are setting timing. If you are a tooth off on the distributor, it will do it, and sometimes the drive gears wear. Its not unusual to have to grind the clamp to get a couple of degrees out of it.

Streetwiseguy wrote: If you can see data on your reader, look for cam offset. It should be as close as possible to zero, anything more than 7-8 degrees will cause crossfire issues in certain conditions. Its adjusted as if you are setting timing. If you are a tooth off on the distributor, it will do it, and sometimes the drive gears wear. Its not unusual to have to grind the clamp to get a couple of degrees out of it.

Are spark advance and cam offset the same thing on the OBD data? I ran the scanner on the way in, and once the engine was warmed up the spark advance was sitting at about 14 degrees at idle...

In reply to Ashyukun:

probably not the same thing. spark advance would be the degrees (I think it is crank degrees because you usually measure on the crank pulley) before piston TDC that the spark fires. 14 sounds about right at idle. Should go up to 30-35 at about 3000 RPM.

I would guess that cam offset is the difference between a reference point in the cam rotation and another reference point in the distributor rotation.

What kind of plugs? I seem to recall that these engines like delcos and that's it.

Does it have the fuel spider under the intake? Those are notorious for weird leaks.

The weird problems with my 350 (4.3 relative) turned out to be bad plugs and wires. Not that old but I've learned to use AC Delco only in GM products. It was suggested that a worn distributor could have been my problem as well. Haven't replaced it though.

Robbie wrote: In reply to Ashyukun: probably not the same thing. spark advance would be the degrees (I think it is crank degrees because you usually measure on the crank pulley) before piston TDC that the spark fires. 14 sounds about right at idle. Should go up to 30-35 at about 3000 RPM. I would guess that cam offset is the difference between a reference point in the cam rotation and another reference point in the distributor rotation.

Yup- the advance did go up to the 30's when driving. I'll have to look through the parameters more closely on the drive home and see if I can find the proper one for cam offset.

As to the plugs- I'd read that the plugs could be a problem so have ACDelco R44LTS6 plugs in it. The middle plug on the driver's side is a BITCH to get to so I didn't want to take any chances that I'd have to pull them.

It does have the spider in the intake system. I'd think that I'd be seeing problems more at higher RPMs/load than at idle were there a leak in it, but I could be wrong.

Clean the throttle body (always step one with gm fuel injection)

If no change make an egr block off by using a piece of aluminum can between the valve and the intake. Sometimes the egr valve sticks open from carbon causing similar problems to what you are experiencing, this will eliminate that.

If neither of these affect it I would take a good look at long term fuel trim for any possible vacuum leaks.

Oh yeah and the leaky spider thing also.

Also the dizzy caps suck on those. Use a good quality cap. Cheapies will cause a misfire super quick.

Like the plugs, I got an AC Delco distributor cap for it when I replaced everything, so hopefully that's not the problem.

When I put the 'new' engine in I cleaned the throttle body quite thoroughly before putting everything back together, so I'm assuming that it should still be good.

If I don't see anything really out of the ordinary with the cam offset, I can see about making an EGR block-off, that should be easy enough (well, not fun in the current weather- but not difficult from a technical standpoint) to do to see if it helps.

Great... after nearly a week of it behaving, the CEL came back on this evening with the same 0507 Idle Speed code.

I reset it and will see how it does. I did note something while sitting waiting for someone who wanted to look at the car- the issue with it missing/stumbling doesn't show up at all when the car is in park- only when it's in drive or reverse and sitting stopped.

Check the wiring harness that runs along the pass side frame rail under the ecu. It runs too close to the exhaust manifold and might have melted a wire or two.

I had a 98 blazer that had a random misfire I changed fuel pump,spider,distributer, and plugs wires. Didn't fix. Changed crank sense too. That truck drove me nuts. I was pulling the ecu when I found the melted harness. Hand to forehead moment. Ran great after wiring fix.

I'll have to take a look at that too when it's not stupid cold out. Mildly annoyingly now that it IS so cold out, I have several people a day texting me wanting to look at it...

Thankfully on the drive in this morning the CEL didn't come back on for the idle speed- but at the same time, it barely gets up to operating temp and into full sensor-controlled mode on my 3 1/2 mile commute. I've been holding out on coming down to $2k on it (which would still be 25% profit, and several people have offered about that), but may just say 'berk it' and sell it for that (I've been quite up-front about the issues and the work I've done on it) so I don't have to fool with it any more.

Unfortunately I'm sick and can't do anything on it at the moment, but when driving around yesterday I was curious and tested something out, with kind of interesting results. Given my previous finding with it not having any idle issues in park, I tried seeing how it did with the parking brake engaged and in drive. It still felt like it was missing/stumbling a little bit, but WAY less than when I was standing on the brake. So, I'm thinking I also need to look more closely at the brake booster line- and hope the booster itself isn't having problems (though given the brakes are a bit soft it may after all...).

I wonder if your converter is not un-locking all the way at idle...

44Dwarf wrote: I wonder if your converter is not un-locking all the way at idle...

That's another interesting possibility, since it only really seemed to happen while in gear.

However- it's not something I'll be worrying about in the end as I sold the Blazer on Saturday with the CEL still on. I pulled up the code on the reader and explained what I knew about the problem but that I also had been driving it for some time without it ever having any real issues and the buyers decided they still wanted it in spite of its issues. I dropped the price a bit because of the problem as I would have spent a modest bit trying to fix it, so hopefully everyone came out of it fairly happy.

i know the spider injection on these trucks can act up and cause weird issues, maybe look into that?