Engine Power Drop Off

[arcmech]

2004 DeVille 4.6 liter Low Vacuum / power loss.

Hello,

I'm having a problem with low Vacuum on a 4.6 L NorthStar. I've left posts at another forum and would like to paste my posts here, see if anyone might have a different view or possibly have run across this same problem.

I'll clean up the posts, try to orginize them as they are answers to other people's questions and comments.

My first post,

2004 Cadillac DeVille 4.6 liter NorthStar

I'm having a problem with power drop off when depressing the accelerator both in neutral and in drive. The problem felt/feels fuel related so I went through the entire fuel system along with checking fuel flow in neutral and in drive comparing the results with a second Deville which is in good condition. Again, all fuel related checks have been made and it's not fuel related.

I checked vacuum at idle which is a bit low, (11-13 in), which drops off near zero when accelerating which prompted me to check basics like compression...(average 160 lbs), exhaust back pressure...(disconnected Cat, no improvement), engine timing with scanner...(8- 10 deg idle, 30 deg revved).

BTW, because of the low vacuum reading I checked for leaks at the intake and throttle body, no problems found.

I've switched a few sensors from the good Deville which might affect acceleration like MAP, MAF,....TPS percentage is within parameters, O2 sensors ok, no improvements.

My next check will be to disconnect the trans at the flywheel incase of a lock-up at higher RPM although the symptoms aren't pointing to this also, since it does feel like it could also be exhaust related I'm wondering if there's a restriction ahead of the Cat, in the exhaust manifolds themselves although I've never ran across this kind of problem before.

After these last checks I'm pretty much at my wits-end. Any thoughts, pointers, input would be greatly appreciated.

Next Post,

As this job progresses it seems symptoms are getting worse. Originally vacuum readings were at 11 to 13 in. with occasionally reaching 15 in. Making a few more checks today, vacuum readings have dropped as low as 10 in. (or less). I switched MAF sensors with no progress so proceeded to check for intake system leaks. I removed and checked the TB, and intake manifold with all parts including boots and hoses checking for cracks, all OK.

I'm leaning towards something plugged in the exhaust system ahead of the Cat, further up in the manifolds or head ports although an exhaust problem of this type I've never encountered and getting to the exhaust manifolds is a bit involved.

I know rings and valve timing are fine confirming this with a compression check with readings of 165-170 lbs average. I'm also leaning towards a defective computer affecting eng. spark timing although my scanner doesn't verify this.

So, a run down of everything diagnosed and found to be in good working order for an engine that stumbles while accelerating and has a low vacuum reading are,

1), Complete fuel check of pump, filter, verifying fuel pressure, volume flow rate, injector restriction.

2), Verifying good working engine, (mechanically), of rings and cam(s) timing with a compression check.

3), Verifying exhaust system for restriction at the Cat/ manifold connection.

4), Check and verify intake system parts for leaks.

Any thoughts anyone may have are greatly appreciated,

Next post,

I removed the intake system to check for any underneath cracks but before dismantling the system, with the engine running I partially covered the TB opening to see if vacuum and RPM would rise but got the opposite effect indicating there were no vacuum leaks....I also removed the EGR valve and checked for any mechanical defects but this also checked ok.

Comparing both my good Cadi and the problem cadi, the fuel trim readings are off.

AS far as ignition coils...when the engine is first started it idles smooth with no misses but with low vacuum. Once accelerated, power drops off in a 'somewhat even' manner with vacuum also dropping to near zero, (but lower vacuum is to be expected as rpm is also dropping). I also removed one coil, installed a test spark plug and watched the ignition spark till the engine died with no spark cut-out.

BTW, the spark was not a sharp "blue" but more orange in color.

Once I accelerate till the engine dies it takes several seconds for a re-start making the problem feel fuel related but the fuel system has been ruled out. It also feels 'exhaust restricted' related given the low vacuum readings. I'll remove the O2's and check for any "junk" build up possibly indicating dirty, restricted exhaust manifolds.

A note: Reading my scanner, on my good Cadi I get a "Load_Pct%" of 2.5 to 5.0% idle. On the problem Cadi it was 25.0% plus. The last scan reading showed a 46.0% at idle but I don't know what this means?? I could use a tutorial on my scanner read-outs but haven't found a good one on-line yet.

After the exhaust inspection I'm pretty much at the end of what I know to inspect with the exception of a faulty ECM not controlling spark timing correctly but reading timing through a scan it reads '8-10 deg' at idle and '28 deg' plus accelerated just before the eng dies.

Next Post,

Yes, absolutely correct, during my checks I have, and still do look at the engine from this angle...a big air pump.

As to valve train, cam / timing chain problems...I've performed a compression test and the readings are 165-170 lbs. average but this of course are static readings, what the engine does dynamically is something else. I've removed the plastic intake manifold to perform a pressure check, (shop air w/ 5-10 lbs pressure plus soap water), but looking at the manifold there are no noticeable cracks or visual defects.

I considered if this engine had VCT and something was failing during run time and not during static testing but It doesn't look like the cams are variable.

I still need to check possible exhaust manifold restrictions but have been dragging my feet on this one as it's pretty involved to remove the manifolds.

Looking at this problem of low vacuum, (10 in idle), what are the possible main causes...

1), intake and/or accessories leak...

2), Cam / valve train timing...

3), restricted exhaust flow...

4), Spark timing-(?)...

With the exception of the exhaust manifolds all other possibilities have been statically checked but will start from square one and perform a re-check starting with the possibility of a cracked intake.

Just a couple of questions,

I mentioned in a previous post about scanner readings, the "Load Pct%" and what this means?

My good Cadi reads an average of 5.0%, the problem Cadi now reads 46%, both cars at idle.

Sorry for such a long first post but didn't want to repeat what was already covered. Any thoughts anyone might have are very welcomed!

Thank's

[BodybyFisher]

You didn't mention whether you checked for codes, did you?

Have you determined if you have a restricted exhaust system? We have seen CATs do this. Have you had any O2 sensor problems over the last few years?

I would borrow or buy something like this, to check if your muffler system / CAT is restricted. This is expensive but you need to use one of these

http://www.autobarn....xw-otc7215.html

[arcmech]

You didn't mention whether you checked for codes, did you?

Have you determined if you have a restricted exhaust system? We have seen CATs do this. Have you had any O2 sensor problems over the last few years?

I would borrow or buy something like this, to check if your muffler system / CAT is restricted. This is expensive but you need to use one of these

http://www.autobarn....xw-otc7215.html

Thank's for your reply,

The only codes were those I set while doing checks,(disconnecting certain sensors). As far as exhaust, I disconnected the Cat, (which is bolted), at the collector pipe end of the exhaust manifolds with no improvements.

Thinking along those lines of an exhaust problem one check I haven't performed are possible restricted exhaust manifolds and /or exhaust ports at the head? The owner reports using about 2 quarts oil every month but there is no visual evidence of any major leaks.

I'm checking the intake manifold for cracks, once this is done I'll re-install and re-run my checks.

Thank's

[BodybyFisher]

If the intake had cracks or you had major vacuum leaks you would set lean codes P0171 and or P0174.

Your problem definately sounds like a restricted exhaust but since you disconnected it at the CAT, that kind of eliminates that. Was the LOAD % the same when the CAT was unbolted?, how did you determine there was NO change with the CAT unbolted?

Unless you see front to rear crossover damage where the crossover is crushed restricting exhaust, I would eliminate that thought, its not likely that the manifolds, exhaust ports are clogged.

If your fuel supply was restricted you probably would set a code also for lean mixture, but check fuel pressure

Have you put a fuel pressure gage on the schrader valve on the fuel rail?, what is the fuel pressure with the key on engine not running and with the engine idling?

The LOAD % is puzzling, and its something to research

[BodybyFisher]

Has anyone messed with the wiring?, could the crank sensor wires be swapped?

Don't worry about the oil consumption, NSs use oil, especially if they are babied, that engine needs to be driven hard, we have a procedure to lessen oil consumption ask about it later when we figure this out

Does this seem like a mechanical problem, seized bearing, etc?

I am speaking with a tech at www.autotap.com, he asked me "what do your fuel trims look like"? Can you see fuel trims with your analyzer?

Check this with regard to fuel trims:

"check for short term fuel trim reading. should be between + and - 8. Then rev and hold engine at 2000 rpm, if number is lower, you have a vacuum leak"

Do me a favor and erase the codes and let the codes set again, and check codes

I know you checked the CAT by disconnecting it, but you can also check your CAT by looking at the pre-and post CAT O2 sensors, post cat will have very similar readings to the pre cat if the CAT is plugged.

Let me know what you are finding

[BodybyFisher]

Have you considered that you have a coil pack out or weak?, the entire bank can be out, how did you determine GOOD SPARK?

[arcmech]

BodybyFisher' date='28 July 2011 - 11:35 AM' timestamp='1311870919' post='193267']

If the intake had cracks or you had major vacuum leaks you would set lean codes P0171 and or P0174.

Yes, there are no intake manifold cracks or leaks after performing a pressure check. I re-installed everything and will continue tests...

Your problem definately sounds like a restricted exhaust but since you disconnected it at the CAT, that kind of eliminates that. Was the LOAD % the same when the CAT was unbolted?, how did you determine there was NO change with the CAT unbolted?

When I disconnected the Cat but before using a vacuum gauge, I started the engine, (ear muffs on!), and accelerated in neutral with the same results of power loss before reaching any kind of high RPM. I'm running some re-checks and this is one I'm doing again but this time with a vacuum gauge installed.

Unless you see front to rear crossover damage where the crossover is crushed restricting exhaust, I would eliminate that thought, its not likely that the manifolds, exhaust ports are clogged.

Visually, the crossover is not damaged...As far as restrictions further up in the manifolds and/or head ports I agree with you, I've never seen carbon build up that bad but I need to rule out all possibilities...although this will be last in the line of checks since the manifolds aren't that easy to get to.

If your fuel supply was restricted you probably would set a code also for lean mixture, but check fuel pressure

Have you put a fuel pressure gage on the schrader valve on the fuel rail?, what is the fuel pressure with the key on engine not running and with the engine idling?

Yes, the fuel system was first checked with a pressure gauge reading 42 lbs running and in neutral. I replaced the filter and re-checked fuel pressure and flow amount comparing the results against the second, "good running" Deville...both tests where the same, (Presure and Flow rate). Up to this point I've pretty much ruled out the fuel system.

The LOAD % is puzzling, and its something to research

This one's got me too!

[arcmech]

BodybyFisher' date='28 July 2011 - 11:52 AM' timestamp='1311871973' post='193269']

Has anyone messed with the wiring?, could the crank sensor wires be swapped?

I don't think so. This auto belongs to my sister and BIL where they reported the car slowly loosing power while climbing then progressively getting worse. By the time I got hold of it, it made one run around the block where it died and I "limped" it home. Now, it probably won't make it out my garage!

Don't worry about the oil consumption, NSs use oil, especially if they are babied, that engine needs to be driven hard, we have a procedure to lessen oil consumption ask about it later when we figure this out

Does this seem like a mechanical problem, seized bearing, etc?

No it doesn't. I thought maybe the Trans might be locking up when applying the accel pedal so I disconnected the converter with no improvements. Once the engine dies it cranks fast, like normal but does take a few seconds to start....just like an engine that's lost fuel pressure although pressure is present. Having given it a compression check it has 165-170 lbs average and cranks fast, normal.

I am speaking with a tech at www.autotap.com, he asked me "what do your fuel trims look like"? Can you see fuel trims with your analyzer?

Check this with regard to fuel trims:

"check for short term fuel trim reading. should be between + and - 8. Then rev and hold engine at 2000 rpm, if number is lower, you have a vacuum leak"

I checked "Live Data" with a scanner comparing readings with me second good Deville and the F.trim Numbers are definitely different. I'll get those numbers to you after I perform a re-check.

Do me a favor and erase the codes and let the codes set again, and check codes

I'll do that once I perform a re-check with the Cat disconnected and using a Vacuum Gauge. I don't anticipate any new results re-checking the Cat but just to make sure I didn't miss anything the first time I checked then I'll run and try to set some codes.

I know you checked the CAT by disconnecting it, but you can also check your CAT by looking at the pre-and post CAT O2 sensors, post cat will have very similar readings to the pre cat if the CAT is plugged.

Let me know what you are finding

Once I've re-checked the Cat I'll run the engine and get some scan readings and post later.

Have you considered that you have a coil pack out or weak?, the entire bank can be out, how did you determine GOOD SPARK?

Good question...Starting the engine, it idles somewhat ok, no major misses. I pulled the L/B, No. 1 coil, installed a dummy plug and watched the spark till the engine died with no spark cut-out. A couple of thoughts crossed my mind when doing this,

1), The spark never cut's out but was not sharp blue in color...more orange.

2), Since the computer controls ignition timing is the computer either retarding eng. timing and/ or cutting pulse-width at the injectors??

As far as engine timing, scanner readings show ignition timing at 8-10 deg idle, and 28-30 deg till the engine dies so I don't think it's timing related.

As to possible pulse-width cutting out, when the engine begins to stumble, applying the accelerator I "squirted" raw fuel into the intake with results of the engine stalling quicker...

As to the coil packs...One fortunate thing is my Brother-IL wants to R&R the eng-trans package which is sitting here in my garage so I can borrow parts from this engine to try.

So far, with the second Deville plus the engine-trans package I can borrow parts to confirm or eliminate possible problems but one item I don't have is a second ECM, (The second Deville is a different year). The Vacuum/ engine problem seems mechanical but is it possible this is a computer problem?

Thank's body!

[BodybyFisher]

Very good interaction here, good give and take of info!

I want you to load test your battery also, make sure it is capable of providing sufficient current to drive the coil packs.

Do some testing you discussed above and lets see what happens, maybe this is a coil pack

A few years ago, I had a bad battery, now I know its a 91, but the battery was bad, and as soon as I stepped on the gas, the engine died. The battery could not keep up with the current demand. keep that in mind.

[Ranger]

Yes, the fuel system was first checked with a pressure gauge reading 42 lbs running and in neutral.

How's the fuel pressure when you accelerate?

I checked vacuum at idle which is a bit low, (11-13 in), which drops off near zero when accelerating

As it should.

Have you checked for a rip in the plenum (the rubber coupler between the TB and the manifold). It's rather common on the 2000+ engines and usually rips on the bottom side where it can't be seen. Spray some Brakleen around the bottom while flexing it with your finger or such and see if the RPM changes.

[arcmech]

Very good interaction here, good give and take of info!

I want you to load test your battery also, make sure it is capable of providing sufficient current to drive the coil packs.

Do some testing you discussed above and lets see what happens, maybe this is a coil pack

A few years ago, I had a bad battery, now I know its a 91, but the battery was bad, and as soon as I stepped on the gas, the engine died. The battery could not keep up with the current demand. keep that in mind.

Good point Body,

A few years back I worked for a transit agancy where we go a shipment of CNG buses which had coil-pack, distributor-less ignition. I remember one bus developed what seemed to be a fuel problem, cutting out while accelerating. Our checks revolved around the fuel delivery system while at the same time making sure ignition spark was not cutting out. Long story short, it was the coil pack causing all the problems.

Question;

Since the ECM controls spark timing, (coil packs), could it be a possible computer problem and showing up as low vacuum?

I'll do a few checks and get those scan numbers to you.

Thank's Body.

[arcmech]

Thank's for your post Ranger

Yes, the fuel system was first checked with a pressure gauge reading 42 lbs running and in neutral.

How's the fuel pressure when you accelerate?

As far as I can raise RPM the pressure stays constant. The entire fuel system, (fuel pressure, flow rate, including injectors), have been checked. Up to this point the F/S has been ruled out.

I checked vacuum at idle which is a bit low, (11-13 in), which drops off near zero when accelerating

As it should.

Absolutely as it should but this engine drops to near zero because the engine's about to die. Vacuum readings now are plus-minus 10 in. at idle. When accelerating the engine RPM rises but quickly stumbles while vacuum drops sharply.

Have you checked for a rip in the plenum (the rubber coupler between the TB and the manifold). It's rather common on the 2000+ engines and usually rips on the bottom side where it can't be seen. Spray some Brakleen around the bottom while flexing it with your finger or such and see if the RPM changes.

The intake system including the manifold have been checked. Removing and pressurizing the manifold and using soap water showed no leaks, cracks or defects. All hoses, boots, plastic tubing have been ruled out either visually, or with carb spray to confirm.

Like the fuel system, up to this point the intake system has been ruled out....

although as soon as I get a hold of one I'll use a smoke system to verify but all looks good.

[BodybyFisher]

I am not saying that it cant happen, but we have not seen many PCM problems.

You observed what you felt was a weak spark (orange). What I would advice is to get a "known good" coil pack and swap it in. That is unless you can get one of these

http://www.sjdiscounttools.com/wae76562.html

[arcmech]

I am not saying that it cant happen, but we have not seen many PCM problems.

You observed what you felt was a weak spark (orange). What I would advice is to get a "known good" coil pack and swap it in. That is unless you can get one of these

http://www.sjdiscounttools.com/wae76562.html

Thank's Body, I have something similar. It's the length and thickness of a writing marker, adjustable, but made for plug wires. I'll see if it lights up if I set it to 'high sense' and touch the tops of the coils.

[arcmech]

2004 DeVille Up-Date...

Performing re-checks of the intake, exhaust, and ignition systems I first started by using smoke to verify the intake system which did check all OK.

Next, I re-checked the exhaust system using a vacuum gauge and disconnecting the Cat at the manifold(s) collector.... running the engine vacuum readings jumped to 16-17 in. indicating a restricted Cat which I mis-diagnosed the first time around.

The engine still had a miss/ stumble while accelerating which felt spark related and was part of the reason I missed the plugged Cat the first time around. I'd thought the power drop off and engine miss were one in the same problem but now see the engine had multi-problems affecting performance.

I next switched the plugs and coil packs borrowed from a spare engine package and welded a temporary straight pipe in place of the Cat...the Caddy came alive, no problems at all with good power / acceleration even under climbing demands. The Trans feels tight and smooth, something the owner thought was one of the problems.

Over the next few days I'll install a new Cat, make a few home made tools, (coil driver and smoker), then go on to the next job!

I want to give thank's for those who posted with ideas along with special thank's to BodybyFisher for all his help and input!

Regards,

arcmech.

[BodybyFisher]

Glad to help, let us know how this turns out

[BodybyFisher]

Glad to help, let us know how this turns out