I'm about to give up!!!! Engine miss

[tommyjack]

92 STS 4.9 with a engine miss at all rpm ranges and engine temps. Very poor gas mileage. Went from 27 to 16mpg. No codes. No smoke. About 70,000 miles on motor. I have replaced the following:

Cap

Rotor

Plugs

Plug wires

Distrubutor

Both O2 sensors

All 8 Injectors

Rodded out the throttle body tubes

Checked the EGR valve, engine dies when valve bottom is pressed in(means it's good right?)

What am I missing here? Low fuel pressure? TPS Sensor? What ???

Thanks,

TJ

[Scotty]

Pressing upward on the EGR will cause the engine to stall. Do you get any codes?

Have you done a compression test, low compression will cause a miss. Don't assume that when you replaced the plugs that you didn't crack a new one putting it in, listen for arching, look at the engine while it is running in the pitch dark for arching. Try pulling one wire out of the distributor at a time and see which one has the least impact on the engine, if you find one that has very little impact on the idle, and the compression is GOOD in that cylinder, you may have an electrical problem that is causing the injector not to fire (that MIGHT set code, but I am not sure). Do you hear any valve train noise? When you say engine miss, it means to me that the engine is not smooth at idle, like one or more cylinders is not firing.

Have you considered that your CAT is clogged and you are suffering from back-pressure? I don't think that would normally cause a MISS however...

[Bruce Nunnally]

Swapped two plug wires by mistake? (

Check here: http://www.caddyinfo.com/howto.html

especially

http://www.caddyinfo.com/howtostall.htm

[tommyjack]

No cats on car. replaced plugs twice, I do see some arking at night even with new plug wires. I don't think I have switched plug wires around. As far as arking goes, where can I buy the silicone to spray on them to stop the arking? I does miss a little at idle. No valvetrain noise. Tried removing one wire at time but that really didn't tell me anything. Wouldl putting it on an engine analizer do any good?

TJ

[tommyjack]

No cats on car. replaced plugs twice, I do see some arking at night even with new plug wires. I don't think I have switched plug wires around. As far as arking goes, where can I buy the silicone to spray on them to stop the arking? I does miss a little at idle. No valvetrain noise. Tried removing one wire at time but that really didn't tell me anything. Wouldl putting it on an engine analizer do any good?

TJ

[Scotty]

No cats on car.  replaced plugs twice, I do see some arking at night even with new plug wires.  I don't think I have switched plug wires around.  As far as arking goes, where can I buy the silicone to spray on them to stop the arking?  I does miss a little at idle.   No valvetrain noise.  Tried removing one wire at time but that really didn't tell me anything.  Wouldl putting it on an engine analizer do any good?

TJ

After hearing your description of miss, describe in detail what a miss is to you. A miss to me is a rough idle because the engine is running on less than 8 cylinders.. Not a little, a definate miss because of a cylinder is not firing.

Now that I heard that, check for vacuum leaks. And you need to check the compression. A little corona around the plug boots is ok. Those are new wires im sure they are fine. I was more concerned with a cracked plug. I will assume that you set the gap on the plugs, I have seen some think that they come set.

Did you check for codes? Was all this work done at the same time?

[Logan]

There should be zero arcing at night.

The '92 did have some issues with the main engine ground mounted on the engine block very near the starter.

Ignition...hmmm....some ideas...the ignition module and AC pickup coil terminals can go 'green' after this period of time.

Hmmm....looking back...I see you changed the distributor.

Check the engine block ground, then check the battery cables for being green. Then I would drop and tap test the ECM.

[Scotty]

Logan, my statement about the corona came from our friend:

QUOTE (bbobynski @ Feb 19 2004, 12:01 PM)

There will ALWAYS be a light show with any high voltage system in the dark. The purpleish glow is called "corona discharge" and can be seen on almost any Northstar in a dark place, especially on a humid night. It is just the magnetic field created around the wires ionizing the atmosphere around the wires. Perfectly normal and not a sign of anything wrong with the wires.

Arching of course is a different story, Mike

[Ranger]

As far as arking goes, where can I buy the silicone to spray on them to stop the arking?

I'd be leary of using silicone. "The Man" once said that silicone is instant death to an O2 sensor.

[jfreezn]

The bad fuel mileage makes me think your fuel pressure regulator is torn internally and is leaking fuel through the vacuum line to the intake. This will not set a code.

Jim in Phoenix

[Scotty]

The bad fuel mileage makes me think your fuel pressure regulator is torn internally and is leaking fuel through the vacuum line to the intake. This will not set a code.

Jim in Phoenix

Good point Jim I totally over looked that. I was confusing this thread and this thread last night

http://caddyinfo.ipbhost.com/index.php?showtopic=7317

[PAUL T]

I was thinking FPR myself, it is easy to check. Just take off the plastic cover and pull the vacume line off the FPR and turn the key on (engine off) to pressurize the fuel system and look for fuel coming out of the FPR. If there was NOT any fuel, it is o.k.. Mine smoked a lot but mine was leaking alot though. On my 92 Deville the FPR was a pain to change.

[Ranger]

I was thinking FPR myself, it is easy to check. Just take off the plastic cover and pull the vacume line off the FPR and turn the key on (engine off) to pressurize the fuel system and look for fuel coming out of the FPR. If there was NOT any fuel, it is o.k..

Paul,

While that is usually true I found out that it is not always the case as evidenced by this thread.

http://www.cadillacforums.com/forums/showt...3&highlight=FPR

[PAUL T]

Paul,

While that is usually true I found out that it is not always the case as evidenced by this thread.

http://www.cadillacforums.com/forums/showt...3&highlight=FPR

Good point Ranger! Nice reading.

[tommyjack]

OK. I put a pressure gauge on it today and it read 36 PSI. The rough idle/miss is starting to come and go now for some reason. 98% miss 2% OK. I changed out fuel filter but nothing changed. Checked the grounds, OK.

One thing about the pressure reading: Turn the key on and it read 38 but began to drop immediatley to 20 PSI. Stared it up and it went back to 36 PSI. Is this normal? Scotty on this site (has been a big help) said the manual says 34-38 PSI is normal. Auto zone says 40-50 PSI. I tend to believe Scotty but now I'm wandering. Pulled vaccum hose off of the regulator, no fuel in hose.

Any help would be greatly appreciated.

[Scotty]

After you posted I did some more research. I think the 40 to 50 is when the KEY is ON but the engine is not running, see this chart, I will post further:

[Scotty]

This is from PFI chart A-5, under notes:

[Scotty]

One of the reasons I wanted you to hook up a fuel pressure gauge was to watch it while you had the problem. Have you been able to do that yet? If your fuel pump pressure deviates when the problem occurs it could be the pump but it could also be the fuel pump relay.

[tommyjack]

Where is the fuel pump relay located? I do the FPR test in about 30 minutes and post my findings.

Thanks!

[Scotty]

Where is the fuel pump relay located?  I do the FPR test in about 30 minutes and post my findings.

Thanks!

According to the 91 manual the fuel pump relay is mounted in the relay center behind the glove box, if you can find a similar relay with identical part numbers on it, swap them and make a note of which ones you swapped. Mike

Check this link also

http://www.borgwarnerbrand.com/techtips/19taptest.html

[tommyjack]

Switched the relays around: miss still there

Unhooked the pressure regulator: Pressure went to 43/44 PSI and held steady at idle and under load: miss still there.

Any body got a match???? (LOL)

I am really at a loss on this one.

[tommyjack]

Under the tree on chart A-5 there is a block: WILL NOT HOLD PRESSURE.

Is it supposed to hold a constant pressure with the key off?

[Scotty]

Do me a favor and itemize what you have done/replaced so far, Mike

This information is from a good old friend, its related to a 93 NS but the concepts are related:

Normally, not a lot is required for a "tuneup" even at 100K as there is little or nothing to "tune up". The dual platinum plugs should be good for a LOT more than 100K but the first thing I would do is to remove the plugs and inspect them carefully as the tiny platinum pads on the center electrode and the ground electrode have been known to pop off with time and miles. Not an epidemic but it has been known to happen. If a pad is missing then that may be your misfire source as the gap has likely opened up. Just replace the plug affected.

Be careful pulling the plug boots as most of the spark plug wire damage occurs when ancient boots are pulled from ancient plugs that have been undisturbed all these years (undisturbed...that's a good thing). It is worthwhile to do a continuity and resistence check of the wires to see if any of the wires were damaged or terminals pulled off. Replace as necessary .

Check the coil towers and wire terminals for corrosion.

Once you have the plugs out for inspection/replacement as necessary it wouldn't hurt to do a compression check just for grins to see if there are any weak cylinders....if your heart can stand the possible answer....otherwise forget it. Seriously, if the ignition system is fine and no trouble is found then this would be the next logical step.

Typically the throttebody needs to be cleaned well before this but that shouldn't cause a rough idle, just maybe some problems with throttle stiction off idle and/or idle speed control anomalies. All you need to do here is to hold the throttle blade open and use carb spray and an old toothbrush and clean the perimeter of the bore and the back of the blade to get the worst of the deposits off.

First, you can do an individual cylinder balance test from the drivers seat using the onboard diagnostic system to pinpoint which cylinder is causing the roughness to help pin it down quickly. Throwing parts at a "tuneup" is really unecessary. If a cylinder can be isolated then start there with the plug inspection and/or swap injectors with another cylinder to see if the miss follows the injector.

I assume you have been replacing the coolant frequently?? If not, then internal engine corrosion may be affecting the gaskets at this point....see the note on compression check above.

The final area of potential problem or maintainence is to remove the EGR valve and clean the passages, the pintle, seat and the shaft of the EGR valve to make sure it is free and operational.

Fog the plug wires with a mist of water in a dark area with the engine running to see if any sparks or leaks light up. Sounds dumb...but is a quick and dirty way to determine the health of the wires....

[Scotty]

Under the tree on chart A-5 there is a block: WILL NOT HOLD PRESSURE.

Is it supposed to hold a constant pressure with the key off?

No with the key on, it should hold pressure, if not you have a leaky injector. I would suspect that you would have a gradual drop in pressure with the key off. What happens?

[tommyjack]

It drops from 40 PSI to 0 PSI within about 5 seconds. As far as a leaky injector, I replaced all 8 last weekend.