Hello again! Such a cool forum, anyways, I did in fact change my intake gaskets, was really no big deal, had no snags, fired up and ran great. fast forward to a week and a half ago. Noticed some VERY slight hesitation when taking off from a start. Then a shuddering or jerking as if it where misfiring or the torque converter was locking/ unlocking at highway speeds. the ses has only come on once, yesterday morning, on my way to work. it was on when I went to lunch and went off while on my way home from work. I scanned it this morning and got the p0300. No other faults. I have the freeze frame values if any one can help interpret them. I can't think of anything else to add. I haven't had time to diagnose anything properly yet, but I am open to suggestions. Thanks so much...abslt tps:10.2, rpms 1970, load 15.7, maf 3.52, map 15.4, temp 189, stft1 3.9, ltft1 12.5,stft2 1.6, ltft2 12.5, 60 mph. By the way, it's been a solid month or so since I replaced the intake gaskets. Thanks all.

 

You say you changed the lower manifold gaskets, did you have the "camshaft retard" adjusted after reinstalling the distributor? A P0300 is usually caused by a fault in the secondary ignition system but can also be caused by incorrect "camshaft retard" adjustment, worn distributor gear and or distributor shaft bushings.

 

No i did not, but once I turned the oil pump gear to align with my timing mark on the distributor, It fell into the exact place that it was and the crank was never turned. The vehicle started immediately as in no timing issues what so ever. Now, with that said, I did notice some wear on the distributor gear.

 

OK Fuel Trim (STFT1 and STFT2) is the computer's response to adjust the air/fuel ratio. In positive corrections, fuel is added to factory calibration to adjust for a lean condition, while negative corrections respond to a rich condition. ±10% is generally the limit of normal, but typically is much closer.

From your freeze frame, one can see that the long term fuel trim is +12.5% on both, indicating that the engine is experiencing an overall pretty severe lean condition at both sensors. However short term fuel trim was only 3.9% and 1.6% at time of the freeze frame so that sounds a bit of a conflict to me. Also to note is that both sensors appear to be affected pretty much the same. So whatever is a problem is common to multiple cylinders and is not limited to (for example) an oxygen sensor.

Display live short term fuel trims on your scanner and note the values at idle and at a steady 2000 rpm. If fuel trims are high at idle, but decrease significantly at 2000 rpm then check for vacuum leaks. Just spray carb cleaner (flammable kind) around the intake and pleanum sealing areas while watching fuel trim. If the values were high and then go down when you spray an area you have found a leak.

If no leaks are found then get the proper equipment and do ALL the tests in the Captain's thread on Fuel Pressure testing (top of this forum) exactly as described (not just part of them). You can borrow a nice high pressure fuel pressure gauge at no cost at many auto parts stores (deposit required). The other tools to block off fuel lines as described will need to be made. Post your results of all the tests, as well as if your spider has been upgraded or not to MPFI. Please note that the upgrade AC/Delco MPFI spiders are known to regulate the fuel pressure to 58psi.

If all is well with the fuel system, move on to verifying the ignition system. Note that cross-firing inside the cap is a known issue with Vortec distributors. This can be caused by a poor-quality cap and rotor, bad plug wires, or the rotor not pointing where it should at the time the PCM chooses to fire the coil. For this reason we recommend a new AC/Delco cap and rotor be automatically installed. The way to check if the rotor is pointing to where it should, is by monitoring the cam sensor retard (CMP retard) live data on the scanner with the engine running. This should read 0° ± 2° or there is a problem. Lining up the rotor visibly with the #6 on the distributor gets you on the right tooth with the factory locked hold down, but that's about all it guarantees. A common problem that can cause the cam sensor retard to be off is a worn distributor gear (more common) and/or worn timing chain (less common).

You can check the distributor gear in place by holding the rotor down so it doesn't ride up on the helically cut gear, and then wiggle it back and forth. There should be zero play. Better yet is to remove the distributor and look at it. Timing gear slop can be checked by watching how many crank degrees it takes to move the rotor after reversing crankshaft direction.

Spark plug wires should be inspected inside both boots for carbon arcing, and checked with an ohm meter. More than a couple thousand ohms is not good.

Spark plugs should not have deposits on them and should be gapped properly. I'm going to go out on a limb and state that contrary to popular opinion, platinum-tip spark plugs are really not necessary. They extend the life of the spark plug, but good old fashioned ones will perform just as well (better) but for a shorter time. So spend your money as you see fit. Kind of like using full synthetic oil - I don't need it since I'm going to change in 3000 miles anyway.

Last thing we need to do before starting any P0300 diagnosis in earnest, is to address whether or not the crankshaft sensor has been disturbed since the last crank sensor relearn (if any has ever been done). If it has been removed/reinstalled or the timing cover disturbed then this is a likely cause for a P0300, but not likely for an actual miss. So do you know if it ever has been disturbed?

OK - that should keep you busy for a while. If you have any questions about anything I have said, please ask before starting.

Regards,

Les

 

Oops! I missed out the the last two posts while writing mine above. Jump directly to the distributor gear if you noticed wear there. If your gear is worn, you can get an entire aluminum replacement distributor from Skip White for $41.50 + ship (I would put AC Delco Cap & Rotor on it). Alternatively you can replace just the gear with local parts (make sure it is not installed 180° out on the shaft as 13 teeth makes this not equivalent).

http://www.skipwhiteperformance.com/...x?Item=6671-BK

Almost forgot to say that a P0300 stored without SES light on is a pending code. P0300 requires problem to happen again before turning on the SES light.

 

I was referring to the "CMP Retard Offset", In order to fine tune the camshaft sensor and lock it down in the fully correct postion you will need access to a scanner which is able to view live data and read the CMP Retard Offset data from the PCM. There are a few scanners that can do this, however I do not have an all inclusive list of them, the most common ones that are used for this are the TechII from GM, and EFILive. If you do not have a scanner that is capable of this, you have two options, you can get one, like the EFILive Scan Tool or Scan-And-Tune, Or you can take the truck to a shop which has access to the proper scanning equipment to have them set it, this should only take a short time to perform, however most shops have a minimum labor charge which can be quite expensive. Because there are different scanners which are able to read the CMP offset, the instructions below will be generalized and not specific to any one scanner.

With the ignition OFF, install a scan tool to the DLC.
Start the engine and bring to normal operating temperature. IMPORTANT: Cam Retard Offset reading will not be accurate below 1000 RPM.
Increase engine speed to 1000 RPM.
Monitor the Cam Retard Offset.
If the Cam Retard indicates a value of 0°+/-2°, the distributor is properly adjusted.
If the Cam Retard does not indicate 0°+/-2°, the distributor must be adjusted.
ADJUSTING PROCEDURE


With the engine OFF, slightly loosen the distributor hold down bolt. IMPORTANT: Cam Retard Offset reading will not be accurate below 1000 RPM.
Start the engine and raise engine speed to 1000 RPM.
Using a scan tool monitor Cam Retard Offset.
Rotate the distributor as follows:
4.1. To compensate for a negative reading, rotate the distributor in the counterclockwise direction.
4.2. To compensate for a positive reading, rotate the distributor in the clockwise direction.

Repeat step 4 until 0°+/-2°, is obtained.
Turn the ignition OFF.
Tighten the distributor hold-down bolt to 3 Nm (25 lb. ft.).
Start the engine, raise engine speed to 1000 RPM and recheck Camshaft Retard Offset.

 

If you want a cheap scanner setup to monitor cam sensor retard and pretty much any live powertrain data on our Blazers, I recommend Car Gauge Pro for Android (Google Play Store) and the BAFX Bluetooth OBD2 adapter (from Amazon). About $35 for both. Then for another $5 get Torque Pro. If you don't have an Android and only have an iPhone then I guess you are SOL for a good cheap app and OBD2 adapter!!

 

Thanks, guys! It appears I have some stuff to do! so, I'll get a distributor gear,check for vacuum leaks,fuel pressure test, spark plugs/ wires (the cap is new ac delco. As far as I know the timing cover has remained undisturbed, but I've only had it since January. I didn't notice any signs of prior work when I changed the water pump though. I am assuming that I will need to set the camshaft off set when I change the distributor? Thanks once again, I am greateful for the free wisdom. p.s. Just out of curiosity,what would good fuel trim values be??

 

Yes, you most certainly will need to adjust or have the Camshaft Retard Offset adjusted after installing a new distributor. Good luck to ya and you're very welcome

 

Ideally, fuel trim (adding or subtracting fuel from normal calibration) would be exactly on the factory calibration if all things are working as designed - so zero is the target. Fuel trims swing up and down constantly - they are dynamic adjustments that are constantly made based on sensor data. This is why freeze frame data is inferior to live data (on what part of the up/down swing did the freeze frame capture the data?). Even long term fuel trim swings up and down - just more slowly.

My personal opinion is that short term fuel trim if monitored live on a properly working system should be staying between -5% and +5% at steady idle or steady cruising speed and swinging back and forth between positive and negative. I believe the official spec is ±10%. Note that if the system cannot adjust the fuel trim enough to obtained the desired fuel mixture, then the infamous rich or lean O2 sensor code is set. You have a lot of fuel trim adjustment to go before that happens.

You're not going to be able to do much definitive diagnosis from a freeze frame, but I do find it curious that both banks showed +12 for long term fuel trim in yours. If it is really biased positive that much on both banks (+12 is center of swing), then it is having to add more fuel to compensate and it would not seem to be an oxygen sensor calibraton problem with both banks off. In reality, it's probably not as severely lean as I made it out to be in a previous post. An electrical miss or fuel pressure problem or a vacuum leak or ??? could affect the overall system enough to throw fuel trim values off and they could just be an artifact of degraded system performance. This is why we want you to verify both the electrical system and the fuel system before worrying much about things such as fuel trim and sensors and the rest when diagnosing a P0300. A lot of things can go back to normal when a seemingly unrelated problem is fixed.

With your statement about the distributor gear appearing worn during intake repairs, I would inspect ignition completely as previously described and repair that first - checking and adjusting the cam sensor retard as required. Either get the required software to do it yourself (I told you the cheapest way in post#7) or plan on paying someone to do it for you. Advantage with getting the software yourself is that you can use it for other things.

I am betting you will be abe to stop there. But if problem persists with ignition system in proper working condition and adjustment, then do the fuel system testing. If problem remains after all is completely verified, then subsequent misfire diagnosis becomes much simpler. Unfortunately people make the mistake of half-assing the ignition and fuel system verifications (somehow they think the tests specified are to difficult or that they just "know" or that they replaced something xx miles ago so they don't have to do them all) and they end up chasing PO30x codes around and around, replacing crank sensors, MAF sensors, O2 sensors, etc.

If you want to see if your Actron CP9185 will do CMP retard and exactly what live data you can display, follow the instructions here. I see it isn't exactly a low-end scan tool. If you haven't figured it out yet from our posts, camshaft sensor retard goes by multiple names. Which one your scanner might use is unknown to me. Focus on CMP or camshaft in the PID name. If you find that you have it, then display the live data after revving briefly above 2000 it and let us know what it reads!

http://www.manualslib.com/manual/164...page=68#manual