Odd engine behavior
06-09-2015, 06:40 PM
#1
Burning Brakes
Thread Starter
Odd engine behavior
Mostly stock 1993 C4 with LT1, 6 spd, MSD ignition, shorty headers. Running Mobil 1 - 5W30. Miles: 44,000
I had an episode of poor engine performance on track Saturday, running HPDE. It was the first of 4 sessions that day (and of 7 sessions for the weekend).
Each session is 20 minutes. For the first 10 minutes of Sat, the car ran fine. I hit the rev limiter very briefly a couple of times (in 4th gear at about 5500 rpm), with no problem resulting. But a few minutes later, the car started running rough like it was starved for fuel or with a miss. Power was reduced to maybe 50%. It recovered for a lap, then happened again and stayed rough and underpowered, so I went to the pits.
I opened the hood and found oil smoking at the rear of the engine block, but couldn't see the source. Oil had also overflowed from the dip stick and from the fill **** in the valve cover. Oil on the dipstick seemed clean and normal, did not appear discolored. I think the Oil temp was 270 as I left the track, but I might have misread it (without my reading glasses). Max oil temp the rest of the weekend (with my glasses) was around 255.
I usually run a half a quart over the full line on the dip stick for the track. I ran this exact oil volume for two days at the previous event without a problem, and did not change or add oil before the problem that I am describing.
I looked for loose plug wires and other clues and found nothing. I started the car back up and it idled smooth and sounded normal. It was cool now, and the overflowed oil was no longer smoking. It was like the computer had reset and everything was fine. At no time did it smoke from the exhaust pipes. No messages were shown on the dash.
I ran 6 more 20 minute sessions and the car ran perfectly the entire time. The only thing I did differently was that I watched my rpm and stayed out of the rev limiter, usually shifting around 5000. That put me into 5th instead of 4th in the straight where I was hitting the limiter earlier.
I had no more oil overflows the rest of the weekend. One other clue: for the last 4 track days (or every track day this year), I will occasionally on start up get a dash message saying "low oil". I turn turn the car off, restart it, and the message goes away.
Could I have a bad oil pressure sensor? Would that affect the computer that sets timing and controls engine performance?
Would 5500 versus 5000 rpm cause this odd engine behavior?
Might it be a bad fuel pump or fuel filter?
And why was it temporary?
Can one pull OBD 1 codes to help diagnose this?
Theories?
I had an episode of poor engine performance on track Saturday, running HPDE. It was the first of 4 sessions that day (and of 7 sessions for the weekend).
Each session is 20 minutes. For the first 10 minutes of Sat, the car ran fine. I hit the rev limiter very briefly a couple of times (in 4th gear at about 5500 rpm), with no problem resulting. But a few minutes later, the car started running rough like it was starved for fuel or with a miss. Power was reduced to maybe 50%. It recovered for a lap, then happened again and stayed rough and underpowered, so I went to the pits.
I opened the hood and found oil smoking at the rear of the engine block, but couldn't see the source. Oil had also overflowed from the dip stick and from the fill **** in the valve cover. Oil on the dipstick seemed clean and normal, did not appear discolored. I think the Oil temp was 270 as I left the track, but I might have misread it (without my reading glasses). Max oil temp the rest of the weekend (with my glasses) was around 255.
I usually run a half a quart over the full line on the dip stick for the track. I ran this exact oil volume for two days at the previous event without a problem, and did not change or add oil before the problem that I am describing.
I looked for loose plug wires and other clues and found nothing. I started the car back up and it idled smooth and sounded normal. It was cool now, and the overflowed oil was no longer smoking. It was like the computer had reset and everything was fine. At no time did it smoke from the exhaust pipes. No messages were shown on the dash.
I ran 6 more 20 minute sessions and the car ran perfectly the entire time. The only thing I did differently was that I watched my rpm and stayed out of the rev limiter, usually shifting around 5000. That put me into 5th instead of 4th in the straight where I was hitting the limiter earlier.
I had no more oil overflows the rest of the weekend. One other clue: for the last 4 track days (or every track day this year), I will occasionally on start up get a dash message saying "low oil". I turn turn the car off, restart it, and the message goes away.
Could I have a bad oil pressure sensor? Would that affect the computer that sets timing and controls engine performance?
Would 5500 versus 5000 rpm cause this odd engine behavior?
Might it be a bad fuel pump or fuel filter?
And why was it temporary?
Can one pull OBD 1 codes to help diagnose this?
Theories?
Last edited by emptnest; 06-09-2015 at 10:14 PM.
06-10-2015, 12:32 PM
#2
What was fuel level, I had mine stuble on track sunday really bad, looked down and yep.. less than 1/4 tank. Not sure about pressure switch, seems plausible. I will dig into FSM when I get home and see if I can find something for you.. Mine is a 96 LT4 but should be same to similar enough for this.
06-10-2015, 12:45 PM
#3
Melting Slicks
Member Since: Apr 2001
Posts: 2,779
Likes: 0
Received 0 Likes
on
0 Posts
Interesting that it would puke oil like that..
In your previous days running the car were you running it up to 5.5K? Pushing oil out of the dipstick tube means you are pressurizing the crank case.. a good bit to push oil out like that from the dipstick
Have you done any compression or leakdown tests? I would suspect rings being compromised in one or several cylinders.
Given that you found oil in these places I'd be surprised if this was an ecu issue opposed to a mechanical - probably rings.
Will
In your previous days running the car were you running it up to 5.5K? Pushing oil out of the dipstick tube means you are pressurizing the crank case.. a good bit to push oil out like that from the dipstick
Have you done any compression or leakdown tests? I would suspect rings being compromised in one or several cylinders.
Given that you found oil in these places I'd be surprised if this was an ecu issue opposed to a mechanical - probably rings.
Will
06-10-2015, 01:11 PM
#4
Burning Brakes
Thread Starter
More info
Fuel level was full.
Hit 5500 rpm a couple of times last year, didn't notice any performance issue. Had not hit 5500 this year till the day described.
Have not performed a compression or leak down test.
Hit 5500 rpm a couple of times last year, didn't notice any performance issue. Had not hit 5500 this year till the day described.
Have not performed a compression or leak down test.
06-10-2015, 06:15 PM
#5
Looking at the FSM for my 1996 it says the Central Control Module (CCM) will display a "LOW OIL" indicator when the oil level is low. For verification it goes on to say wait 10 minutes for all oil to drain then check oil level on dipstick.. Haven't found anything mentioning a reduced power if light is on.
I had my oil 1/2 quart over at roebling road and it puked out the dipstick as well. I ran it about an 1/8 above full mark at Road Atl last weekend and it was fine. Also got a new OEM dipstick which has the green O ring near cap as original dipstick was loose in tube and had a simple cork material for sealing.
I had my oil 1/2 quart over at roebling road and it puked out the dipstick as well. I ran it about an 1/8 above full mark at Road Atl last weekend and it was fine. Also got a new OEM dipstick which has the green O ring near cap as original dipstick was loose in tube and had a simple cork material for sealing.
06-11-2015, 08:45 AM
#6
Melting Slicks
Do you still have the Optispark??? Those are notorious for acting up and then acting normal. Just my experience with them. Your mileage isn't high enough, or at least I wouldn't think so, they start to go south at about 90k, but if you got some oil in it for whatever reason, or it could start to misfire and act up...
06-11-2015, 09:08 AM
#7
Burning Brakes
Thread Starter
06-11-2015, 10:12 AM
#8
Team Owner
Member Since: Mar 2001
Location: Boston, Dallas, Detroit, SoCal, back to Boston MA
Posts: 30,606
Received 239 Likes
on
167 Posts
You can make the codes come up on the newer dashes
http://s242.photobucket.com/user/swi...?sort=3&page=1
That sounds less like an opti error and more of an ignition module problem.
Check the back of your intake & the rear main seal for the oil leak
http://s242.photobucket.com/user/swi...?sort=3&page=1
That sounds less like an opti error and more of an ignition module problem.
Check the back of your intake & the rear main seal for the oil leak