I'm not certain if this subject belongs in this forum or in the Engine/Trans forum - mods feel free to move it if you feel it would be appropriate.

This is a problem with my 2000 Blazer ZR2:

This just cropped up this morning while leaving for work - the idle is very rough and sits about 500 rpm with the AC off. Acceleration is sluggish, pushing on the pedal gives no immediate response. The engine didn't try to stall or feel like it bogged down when I hit the gas, it just doesn't seem to react without a delay. Fuel level is between 1/4 and 1/2 tank. I had to sit at a railroad crossing for a few minutes, the idle felt rougher than my 91 with its Edelbrock cam. I ended up taking it back to the house and driving the Malibu instead. These things always seem to happen when you're already running late! The last time I drove it was on Sunday and it ran normally - mix of city and highway on that trip. I'm not sure what's changed since then.

It definitely sounds and feels like a misfire. Took it down the highway to the gas station and back. When cruising it is resistant to acceleration until it downshifts. Still accelerates well under hard throttle. Doesn't sputter and try to stall when snapping the gas open from a stop or letting off the gas quick from full throttle.

-Plugs are about two years old, air filter around last October, replaced the charcoal canister and both valves last winter. Alternator was tested earlier this year when I replaced the battery.
-Plug wires appear to be in place properly. I attached my timing light to each wire as well - each one caused the light to blink in about the same pattern. So, distributor and cap/rotor appear to be good. Very quick strobe when the light is attached to the coil wire.
-Exhaust has a strong gas smell - normally no odor at all, like smelling a running hair dryer. Tailpipe shakes noticeably, distinct 'chugging' sound as it idles.
-Engine visibly shakes under the hood.
-Vacuum read 18 in/Hg but I could only find ported vacuum so it didn't act right when I tried snapping the throttle.
-PCV valve rattles. You can feel the stopper snap back and forth by covering and opening the hole with your finger while the engine is running.
-Oil is relatively fresh, only about 300 miles since the last change. Light amber color, no visible chunks and doesn't feel gritty. Didn't feel any pressure coming from the PCV valve port in the DS valve cover.

Still no MIL codes. I'm open to any suggestions at this point. This ZR2 still feels relatively 'new' to me as I drove my 91 Blazer since it was new up until 2008 when I purchased this one. As I'm new here I'll explain the name to clear up confusion - I registered with this name on a number of S-Series forums over the past eight years when my 91 4WD Blazer was my daily driver. To keep things simple (and due to a lack of creativity) I just keep using the same name.

Thanks for any help you guys can offer.

I finally got a code last night - P0155: O2 sensor heater circut malfunction (bank 2 sensor 1).

Installed the new O2 and cleared the code. So far no difference. Still chugging and shaking at idle, exhaust smells like gas and bad hesitation when you hit the gas. Surging seems even more noticeable now when holding the throttle steady.

A bit more info-

I don't feel comfortable trying to do a full ODBII drive cycle - I really don't want to take it on the highway to get it up to the required speeds. I started it up and let it idle in D with the AC and rear defroster on for over two minutes - I let it go until it reached operating temp. It was idling noticeably rough as expected. I then put it in P and gave part throttle to get the RPM's up to 3000 - no audible misfires, backfires or sputtering. I let off the throttle and then shifted back into D and let it idle. Now it was idling extremely rough, shaking the vehicle visibly. I let it keep idling like this (RPM's about 500). It sounded like it was straining to keep running. Sooty smoke was visible coming from the exhaust - along with the gas smell. Obviously there's a very rich condition going on here but I'm not sure what to check. No codes yet. I'll try this one more time once it cools off so it can do a full warmup cycle.

It still starts up with no trouble so it doesn't seem like the fuel pressure regulator is leaking. I haven't got any misfire codes so I don't think the poppets are sticking (but I don't really know all the symptoms).

Unfortunately I called the local shop to see if they could lend a hand - they have no one on staff to do diagnostics until after the holiday weekend.

As I mentioned, I have the MFI spider kit but I've been avoiding the chore of installing if until a problem arose with the CSFI injectors.

Any other investigation I can do to help narrow this down?

Why not start with actually diagnosing the code given rather than throwing parts at it...? The code doesn't automatically mean that the sensor needs to be replaced...

P0155 - Heated Oxygen Sensor Heater Circuit (bank 2, sensor 1)

You could also mist down the plug wires with water in the dark to see if spark is escaping. Wires can 'look' and in fact be new and still leak spark. Did you actually remove the cap to check it or just look at the outside?

Wow, thanks for such a warm welcome. You're making me reconsider asking for help here. Based on that troubleshooting info I'm lead to believe my assumption of a bad O2 was correct as I've made two cold starts and let the truck run for more than the specified two seconds and the code has not returned.

The cap and rotor have about six months on them - I replaced them last fall. No noticeable deposits or carbon tracking in the cap - terminals have minimal deposits. I went and sprayed the wires for the heck of it. No light show from the coil or wires.

A new symptom has developed when I let it sit and idle - dieseling. It sputters and keeps running for a second or two once the key is off. After this it's difficult to restart - similar to when I had a leaking fuel pressure regulator. I'm still wondering if the injectors, poppets or lines aren't leaking inside the intake plenum.

It sounds to me that you may have a leaking FPR or your spider assembly may be leaking a fuel pressure test will tell you more, it is defiantly getting way too much fuel... and as far as the warm welcome from Kyle, he is just trying to get you to use the search function, there is a lot of valuable information here at your finger tips if you are willing to search

I recently had the exact same symptoms with my 2000 Blazer and along with Swartlkk's advice, finally tracked the problem down to a faulty distributor cap that I had just purchased (brand spankin' new) about two days earlier. Nothing that a warranty couldn't replace.:icon_doh:

I pulled the plugs this afternoon to see if the issue is isolated to just a few cylinders. I can't seem to find my fuel pressure tester.

2, 4 and 5 are black and sooty.
6 looks about normal but has some soot on the back side of the insulator in line with the side electrode.
1 and 3 look normal.

I couldn't see a great deal through the throttle body with my little mechanic's mirror so I can't say if any areas look washed clean.

I suppose it's time to pull the intake plenum to investigate further.

Yeah I would pull the plenum, leave the spider all hooked up and turn the key on and see if you can see any leaks. Also look to see if one side is washed clean, that is a good indicator that there is a leak

I finally replaced my missing fuel pressure tester, it's sure acting like a fuel leak-

Putting to key to run without starting it jumps to 66psi while the pump is running.
It drops to 58psi when the pump stops.
The pressure slowly drops as you keep watching the gauge, 1-2psi in 10 seconds.

Real similar to when the regulator was leaking. Since the rich condition seems to be on three cylinders perhaps three of the poppets are leaking?

I tested the TPS as well. It's getting 5.0v and the sense line shows gradual increase from 0.40v to 4.50v as I'm used to seeing.

I'll pull the intake plenum tomorrow morning and see what I find.

Hopefully the issue is corrected now.

I pulled the intake plenum this morning and saw this:

Attachment 38585

I dug out the MPFI parts and finally did the swap. MPFI next to CSFI spider.

Attachment 38586

Here's a closeup of a 'good' poppet - one from a cylinder that didn't have a sooty spark plug:

Attachment 38587

Here is a 'bad' poppet. Can't really tell the difference.

Attachment 38588

I see some clean spots on the injector retainer plate and on the regulator housing clip, so it looks like that's where it was leaking from. I didn't give it a great deal of attention as I wasn't looking to repair it.

Attachment 38589

Regulator on the new spider, for comparison.

Attachment 38590

Cleaned up the lower intake manifold a bit before installing the new bracket and fuel meter assembly.

Attachment 38591

The GM TSB recommended routing the #3 injector line behind the #1 to avoid interference with the upper intake plenum. Those hoses are a bit fiddly to get them right where they need to be. Fortunately they seem to bend more than you think they will.

Attachment 38592


Started up on the first try and puffed out a bit of smoke - burning off all the carb cleaner I used! After that the idle settled down. No shaking, no audible misfires, no codes. I took it on a test drive on our back road and then on the highway. Running a lot smoother than I can ever remember - idle quality is better than what I've considered 'normal'. Went through an OBD II drive cycle - kind of a pain to find a stretch of road where you can coast from 55 down to 20. No codes yet. Also, the smell of unburned gas in the exhaust is gone - just smells like hot air as it used it.

It feels a bit sluggish in the low end, I'm wondering if it's just the bad plugs as I haven't changed them yet. I picked up new ones last night but this was enough work for one day, I'll swap them tomorrow morning and see how it goes.

Well glad to hear that you found the issue :)