Oil pressure jittery; spontaneous misfire; then engine cut off
 

I was at a stop light the other day, just started driving my Blazer that day, and I noticed the oil pressure gauge was acting jittery. At idle on a hot engine it would bounce around 20 PSI and jitter right there. I also felt the engine misfire a bit. So I gave the engine a good rev there at the stop light, brought it back down to idle, then the oil pressure would drop very close to 0 and the engine felt it was misfiring. I revved it again, brought it back down to idle, then it suddenly cut off. What could cause this sort of behavior?

My 2002 S10 Blazer has 170k miles on it, and this is the first time ever it had done this. I just replaced the spark plugs a few weeks ago.

A lot of things can cause this. Fuel delivery, ignition, bad sensor, air delivery, bad cat, etc. If you want to figure it out without the parts cannon then you will need to do a proper diagnosis. A lot of guys here go straight to swapping parts but that can get expensive.

How was your oil pressure before this?

What did the plugs look like?

Have you done a compression test recently?

Do you have a scanner with live data capability? If so, any codes? If you want to do your own diagnosis and repair, this is pretty essential and not that expensive. The key is live data and a few special things like misfire counter and cam retard. There are members here that can hook you up on the cheap.

Depending on the answers to the items above, you should do the fuel pressure tests in the sticky above.

George

It would always bounce just a little while idle/hot around 20. That's been normal never since I've owned the vehicle. This instance the other day was far more pronounced than ever.

https://cimg3.ibsrv.net/gimg/blazerf...e17ada9392.png


I've never done one before. I suppose I should look into it.

I have a BlueDriver I can use to pull live data. I'll collect some data over the next few days. There are no code though. At least nothing relating to this. I have ABS module errors, but that's a different thing I'm still trying to diagnose.

GeorgeLG, you mentioned bad sensor. Could it be MAF sensor or oxygen sensors? I don't think they're bad, but they are the original factory parts. I guess pulling live data would determine that.

It sounds like your oil pressure was normal for an engine of this age before hand so the oil pressure jitters are probably related to the engine stumbling.

Your plugs look fairly normal for an older engine.

Live data will help you get to the bottom of this. Start with the misfire counters by cylinder to see if its random or specific to a cylinder. Look at fuel trims to see if they are out of bounds. If nothing there then you can go through the sensor data. There are many possibilities there: O2 sensors, MAF, MAP, TPS, ECT, crank, cam, ....

Don't forget fuel pressure. How old is the fuel pump?

George

Fuel pump is new. Was replaced about 2 years ago, and a new fuel filter was installed.

What brand?

George

ACDelco.

OK good, a reliable choice.

George

Here's some live data I pulled yesterday on a short drive.

The first set CSV is me driving around my neighborhood with speed bumps. Engine RPM may seem high compared to vehicle speed because I was driving around only in first gear trying to diagnose a vehicle shaking/balance problem for another thread.
https://docs.google.com/spreadsheets...it?usp=sharing

The second set is me hitting the adjacent highway for about 5 minutes then turning around.
https://docs.google.com/spreadsheets...it?usp=sharing

Is this the right place to post this data, or should I seek out someone how knows what to look for?

Some screenshots from my BlueDriver
https://cimg7.ibsrv.net/gimg/blazerf...54ccf5715a.pnghttps://cimg1.ibsrv.net/gimg/blazerf...cddfdb3b70.png
https://cimg2.ibsrv.net/gimg/blazerf...962118fa02.pnghttps://cimg5.ibsrv.net/gimg/blazerf...9c72030756.png
https://cimg6.ibsrv.net/gimg/blazerf...d5b4bf6344.png

I am glad to see you start learning how to properly diagnose your engine with analytical tools and you have started with many good sensor choices but there is a lot going on here and it would be helpful to get some controlled simplified data points.

First: Please comment on how the truck is running during these data grabs. Is it stalling or running rough at any point in this data?

There are a number of bizarre readings in that tabular data but I don't know how much credibility to place on those transient readings and your capture process. They might be helpful later if there is an intermittent problem that is not obvious from more traditional dashboard monitoring or if you happen to be catching the data when the engine starts running rough. For now, I would focus on the dashboard data as in the screen grabs at a couple of steady state conditions - Dead cold/engine off/key on, warm idle and warm 2500. We also need MAF at warm 1700 because I have data on our engines there. Get the digital readings at those points but also look at graphed data to see if there are glitches at steady state RPM's indicating that the sensor/wiring/5v reference is flaky and starting to fail.

RPM: You should always display RPM's in your data set including in the dashboard gauges. I cant tell for instance what is going on with the engine when the O2 sensors stop oscillating and go to a slow ramp. At a steady RPM this would be abnormal but if RPM's are changing under say acceleration or a snap throttle then the readings might be OK.

ECT: Your coolant temp readings look normal after warm up but high initially. The reading should be near ambient with a cold engine, unless it had not cooled down completely.

Fuel trims: You always need to look at short term and long term trims added together for a given bank, mindful of sign. In some of these grabs the data is cutoff. The ones that provide all 4 data points look fine. +/- 10 or less is good, > +/- 10 is marginal, >+/- 25 sets a code. Steady state idle and 2500 rpms would be good here.

MAF: In that tabular data the MAF readings at idle look typical for the 4.3L but are all over the place at higher RPMS. Again grab those steady state RPM's, idle and for MAF - 1700. Again get the digital data but also look at graphed data vs time at a steady state rpm to see if the sensor/wiring/5V referense/etc is acting flaky.

O2 sensors: The readings should be oscillating from near zero to near 1.0, usually from say 0.1 to 0.9 as yours are much of the time. I don't know if those changes to a ramp are OK or not because again, we need RPM data or to confirm if the RPM's were steady state or not. There is a place for snap throttle and full acceleration tests but I would stick to a few steady state rpm data points out of gear in the driveway first. An O2 sensor can still be bad even if it is oscillating as described if it is getting "sluggish" but lets start simple.

TPS: The best first order TPS test is key on engine off and watch the data as you slowly move back and forth from idle to full throttle. Does the data make sense at the extremes? Any glitches? Do it again after the engine is warm, any changes?

MAP: Is the reading near atmospheric at cold key on engine off?

IAT: Is the reading near ambient at cold key on engine off?


I'll do my best to help you but if you feel that you need to share this data elsewhere then by all means carry on soldier. Knowledge is power.


George

Also, if your engine dies and will not restart, you should immediately see if its due to no spark or no fuel pressure. Travel with a fuel pressure gauge and a spark tester/screwdriver/spark plug. Intermittent problems can suck but catching it in the act is the key. Also, if your running graphed data on the key sensors when the problem happens, more clues. Freeze frame data is another path to the answer.

George

ok im new at reading codes i just replaced fuel pump a week ago and my Evap,EGR,and catlyst will not show so failing inspection also what is a good beginner code reader i can use on my tablet
Also it is a 2001 chevy Blazer LT 4dr

The spark plug is WAY too dark for a properly tuned EFI engine. Brown = unburned fuel. Unleaded spark plugs should be clean near white with some lighter orange or yellow deposits. Your brown plug is fouled

If its a 4.3, min. Oil pressure us about 6 psi. 20 is a bit low, mine runs around 30 with 280,000 miles on the clock.

Ok George. I'll work on getting that kind of data.
I've always been able to restart the engine right after it cuts off in these scenarios.

Does the vehicle have to be moving getting data at a constant RPM?

No, just set the throttle in park at the throttle plate or the gas pedal.

George