FieroLT1

I have a '95 LT1. All the goodies, heads, cam, rollers, you know the drill.

Has been running like a champ for many years. But just yesterday, at a stop light, i heard a loud explosion followed by a large cloud of white smoke that engulfed the entire car car. The check engine light immediately came on and the engine started missing. I panicked and shut it off.

Since I was at a green light and cars honking behind me, I tried to start her again, which it did, with a lot of sputtering and missing. For some reason, I was not able to shift into first gear, so I had to take off in second. I made it home just barely with little to no power. My guess is I was in "limp home mode".

Once here, I broke out my AutoXray and scanned the codes to my surprise there are no codes stored. But the codes history had two, 48 = MAF failure and 28 = Pressure Switch Manifold (xray's description) since it should really be Transmission Range Pressure Switch. Plenty of tranny fluid though. I took the MAF off and it looks good to me. BUT! I do see oil in the throttle body by the butterflies. Don't know if that means anything. I cleared all codes, sprayed the MAF with electronic cleaner and started the car again. The check engine light came on again. But again, no codes are stored only the history of those same two.

I am not sure what do do now. Car idles just fine in the garage. No white smoke is coming out either but the exhaust stinks to high heaven, when throttled hard it doesn't pickup immediately like it used to and the check engine is still on. Not sure where to take it from here.
The MAF is a Granatelli and if I remember correctly, I paid over $300 for that sucker years ago, I hate to replace it to find out that is not the problem.

Any ideas?

Fatdog2

Sounds Like when my air conditioner hose blew on another car.It was a big bang and then white smoke engulfed the car.My friend had put too many cans of freon without a gauge and the next day it blew up while in line at jack in the box.

Injuneer

Did you follow up on any of the checks I mentioned in my PM?

DTC 28 sets when the signal from the transmission range assembly switch causes the PCM to see 0 volts on PCM pins C28 (range signal "A") and C30 (range signal "C"). Doesn't have anything to do with the fluid. This would appear to mean that the 5 pressure switches inside the transmission range pressure switch assembly are not functioning correctly. The purpose of these switches is to tell the PCM what gear the transmission is in.

When DTC 28 sets, the PCM defaults the trans to 4th gear, the torque converter lockup is defeated. If the trans is overheated, appears you get nothing, since it will not engage 4th gear.

DTC 48 sets when there is no signal detected from the MAF sensor. The PCM defaults to speed-density method for calculating mass air flow. That means it is critical that the PCM receives a good signal from the MAP sensor, and from the IAT sensor.

Key on/engine off, check for 12 volts between the pink wire and the black/white wire at the MAF sensor harness connector. If you don't have 12 volts, recheck between the pink wire, and a good chassis ground. If you have 12 volts, disconnect the harness connector, and measure the voltage between the yellow wire and the black/white wire. Should be 5 volts.

If you have the correct voltages, it's probably the sensor. If you have a multimeter with a frequency scale, reconnect the harness connector, start the engine, and measure the frequency across the yellow and black/white wires. At idle should see something in the range of 2,500 Hz, and the frequency should increase as you increase RPM.

I wouldn't worry about the Granatelli MAF.... it's really doesn't do anything that a good PCM tune can't do (lean out the target A/F ratio in PE mode), and a tune is cheaper than the Granatelli. I'm surprised your tuner didn't tell you to remove it, since it is intentionally miscalibrated, compared to a stock MAF sensor.

As far as the oil in the TB, pull off the TB top cover and look for a puddle of oil in the chamber on top of the TB, and pull off the rubber line from the TB to the passenger valve cover and look for heavy wet oil. That is a sure sign of either blowby, or a plugged PCV valve.

FieroLT1

Thanks Fred.

Yes, I started following the troubleshooting you recommended. I broke out my '95 manuals and the multi-meter and started jotting down voltages and measuring points.

But since you have laid it out here so nicely, I think I'll just follow this and report back.

Thanks for the help!

FieroLT1

Hi guys,

A quick update on this issue. Per Fred's great instructions, I measured the voltage on the MAF to be 11.79 volts between the pink wire and the black/white and 5.05 between the yellow wire and the black/white wire. So that checked out. However, the Freq reading was about 400Hz at idle and fluctuating all over to 900Hz and down to 100Hz, and upon throttled, actually went down zero. So that was fishy.

The rubber line to the valve cover sure enough had heavy oil in it and the TB top under the cover had a big puddle of oil in it.

So, I guess I'm on my way to get a new PCV valve and a new MAF. Stock this time.
I was due for a tune anyways, so I can have the ECM taken care of there and then.

Thanks Fred for the really good advice. This is the second PCV I get but never have I seen the explosion and the white smoke before. i was sure I had blown the engine! That freaked me out!

Injuneer

You are not home free, yet...... the oil in the rubber hose can be cause by a plugged PCV valve, or by excessive piston ring blowby. Take the oil fill cap off and put your hand near the opening to see if air is puffing out of it.

FieroLT1

Thanks Fred. No puffing at idle. Air coming out of the hole at about 2000 rpm. Is that good, bad or normal?

BTW: The new MAF and PCV are in. Codes are gone, but the throttle seems off. Hope the tune helps that.