Alright so i have a 1998 blazer 4 door 4wd with a auto transmission, 150201 miles. my problem is that my car seems to lurch under load (driving around) it feels like it misses for a second randomly then continues normally. i have a rough idle along with heavy random 'misses' that drop the rpm from 550-600 down to like 400-ish. this can happen anywhere from every couple seconds to every 20-40 seconds depending on things. I've noticed it seems to run rougher with a lower gas tank (anything lower than about 1/2) and it seems to do it more often if the engine is at operating temp or if the a/c compressor is kicked on or its drawing vacuum for in dash blend doors and stuff. i haven't had any problems with it being hydro-locked or not starting. i drove it on the highway for about an hour a week or two ago and it constantly felt like i was dropping in power or the car was braking with the cruise control on. I have checked my fuel pressure (i replaced the pump 10 months ago, no problems that i know of) and i had about 60 psi of fuel pressure with the car off and roughly 54 with it running at idle with the needle jumping all over the place (40-60) once the vehicle was turned off it maintained a constant fuel pressure for 5 minutes and only dropped down to 40 psi once it was left for over an hour. i did a tune up on it a month ago along with cleaning MAF and throttle plate. at one point i believe the oil was over filled just a little bit and i don't know if it was overfilled enough to let the crank hit it and air-ate the oil and cause a oil pressure issue or not. i have not had a chance to put a oil pressure gauge on it as we don't have one at my work although the gauge in the car (which i dont trust) says that at 500 rpm i have 25 psi of pressure and at load about 45. one thing that is notice is when the vehicle has a 'miss' the pressure gauge drops a bit or jutters some. when i get a chance i am going to check compression. I am also getting NO CEL or any codes of any kind. I am in need to figure this out, im worried my engine has a spun or fried crank or connecting rod bearing and is going to seize my engine at some point. Suggestions, thoughts or anything would be greatly appreciated!!!

 

I was having exact set of circumstances (also noticed oil pressure gauge fluctuating) and took it to a shop who said MAF was dirty. After cleaning it I noticed the oil gauge doesn't seem to be fluctuating anymore and problem SEEMS to be solved for now. Only time will tell. Oh yea, I am using a K&N air filter which may have contaminated MAF.

 

Oil pressure sounds fine. Dropping RPMs due to misfire will impact oil pressure, expecially when warmed up at idle.

Does SES light work? System is supposed to detect misfires like you are describing, turn on the SES light, and set misfire code(s). If system isn't detecting felt misfires, then start there.

 

My question to Lesmyer is that if the system isn't detecting felt misfires, you say to 'start there'. Not to sound like a smart ***, but where exactly would one start? If there is no code given, one is sort of left to blindly search around for the culprit. Unless you mean to start to rule out the obvious suspects.

 

have you actually put a scanner on the truck? mine had a "pending code" that did not light up the ses light but was in the trucks computer. i have recently learned some codes need two full driving cycles to light the light. i had no light and came up with a stored pending code.

 

It's only 5 minutes of testing I am suggesting before starting to get into the rest.

Start by verifying that the SES light comes on with the key and then goes out after the engine starts.

Put a scan tool on with the misfire occurring. Check for pending, current, and history codes. Record and then clear them if any are present. See if any come back (with the misfire occurring). Note what codes come back and what type they are. Like Er0ck273 ponts out, some will show pending and not turn on the SES light unless problem continues on more than one key cycle.

If no misfire codes (including pending) are present when you can feel a misfire, then the misfire detection system is not working correctly. You can verify misfire system failure by inducing a dead miss into your engine (pull a wire at the distributor and rig up another plug wire/spark plug and ground the plug somewhere). Then start the engine and live-monitor misfire counts for that cylinder with the scanner. I suppose just check for a pending misfire code on the cylinder you have disconnected would effectively do the same thing if you don't have the misfire PIDs on your scanner. If the misfire counts are not going up for that cylinder, then something very basic is wrong with the system. Usually when a problem occurs, false misfires are detected, not the other way around. But that is why I ask if the SES light works at all. Your misfire system may be just fine - but........

I spent a lot of time chasing a false P0304 (#4 misfire code) in my own Blazer last Fall right after I bought it. Started first day I got it home with a ticking lifter noise and low oil pressure (~10psi) when fully warm - no felt miss. Eventually checked codes and a P0304 and P0650 were present. Turns out the PO had disconnected the SES light at the IC connector to cover up a problem P0304 misfire code during trade in. Fixed the SES light and then it stayed on 100% of the time with the P0304 - yet it ran good with no felt miss at all (just ticking and low oil pressure when fully warm). When I induced a dead miss on a different cylinder and monitored misfire counts, I got nothing (except #4 cylinder continued to be detected as misfiring). So I concluded that the misfire detection system was not working correctly and that sent me in a different diagnostic direction. Reading the factory manual, I came across a list of things that can cause false misfires to be detected, one of which was the thrust bearing allowing the crank to walk back and forth (whacking out the crank sensor as far as beng useful for misfire detection, but still OK for triggering the coil). I (gently) pried my harmonic balancer back and forth and could actually make a clunk noise and see the crank move a little bit, so I knew the thrust bearing clearance was way excessive and I resigned myself to fixing the engine bearings for both the oil pressure and false misfire code. I won't go into the details about how I ended up fixing things twice after that, but I can tell you that a bad rod bearing without the thrust bearing being bad will also cause a false misfire code. Now that all is fixed, I can induce a miss on any cylinder and it will count each misfire correctly in a scanner's live monitor.

Now your problem seems to be different, yet it strikes me as somewhat the same. For some reason you are paranoid about your oil pressure. Are you hearing knocks or ticks? The oil pressure you describe is right on what I now observe with my repaired engine and 5W30 oil. You are feeling misfires when they should be detected by the system (opposite of me). By starting there I mean you should verify that your system can actually detect a miss first, because that is a very basic function of the system that does not seem to be working correctly. Same thing that causes misfire detection to fail might cause your miss. If you misfire detection is verified to be working correctly, then you can eliminate some possiblities out of the box such as the much maligned crank sensor and know that it does not need a crank sensor relearn. I also think that proper misfire detection implies your bearings are good. So take it for what it is worth... I personally think checking misfire detection by inducing a miss is well worth doing before getting into other things. It's really pretty easy.