SHO missing it's punch but no check engine light

[Randall]

Hey everybody,
I recently added a catless y pipe and a 2.5" catback with a resonator and magnaflows to my SHO after first trying it with no resonator, and in the days after this I have noticed a substantial drop in power. The car gets up and goes still, but it takes longer and it even seems to bog down momentarily when under full acceloration. The idle seems normal on the tach at about 850 or 900, but it sounds higher than that and seems to have a slight surge. I don't know if the exhaust mod caused this or if it has been slowly happening. I ran the codes and other than passing ones, it gave me a 542 - fuel pump open circuit, in the KOEO memory. I have a fuel pressure tester and plan on checking it tomorrow, but does anyone have any other ideas as to what could cause this?
I did a 60K upper service two weeks ago with new autolite double platinums and Taylor 8mm wires. It seemed to run great until I started messing with the exhaust, but the 542 code has been there since I bought it three weeks ago.
Thanks!
-Randall

 

[Randall]

I also noticed that the cam seal under the cam position sensor is leaking, and in my other SHO, this went bad without throwing a code, could this cause a loss in power?

 

[Randall]

bump.....

 

[Speedie13]

Well the only bit of advice I have for you is check to make sure your exhaust is bolted in nice and tight and make sure there arent any leaks around the exhuast manifolds and maybe even at the ypipe/catback ****** (if you have thats as opposed to a flex pipe). This may not be the case but my 95 SHO has a nasty exhaust leak somewhere and its not running as strong as usual either. It also for some reason doesnt have an IMRC solenoid either but thats beside the point. My advice....check the exhuast over real well and tighten everything down good....see what that gets you. Hope this helps a little.

 

[CerberuS]

Was the interior of the former cats ok? if they are desintegrated it *might* have clogged the muffs.

 

[Rustynuts]

Check all vacuum hoses too. Maybe your secondaries aren't opening?

 

[Randall]

Yay, the forum is back!
Thanks for the ideas guys!
After running multiple tests on my SHO, I found a few sources of trouble that had caused the conditions above. They were: a bad cam position sensor, although no code ever came up, a completely clogged DPFE sensor sample tube, and last but not least low fuel pressure on idle and start up. All items were replaced. After doing all of this and reseting the idle, the SHO is back, and is now the fastest of the three I have owned by far!

 

[backdaddy]

Sounds just like what my 92 is doing. It sat for a year+,just did the CPS and water pump. It ran like a banshee for 2 days, now its a dog, also doing some popping at low RPM. What is the DPFE sensor tube? Also what did you do about the fuel pressure did you replace the pump? I noticed when bleeding the rack it jest dribbled out. I replaced the fuel filter but it sat for over a year, wonder if its full of crud.

 

[Brett]

Also curious as to youre remedy of the low fuel pressure, pump im assuming?

 

[SASHO91]

I would be willing to bet that the loss of power was due to the FP mainly. I dont see the CID as the problem, as well as the DPFE. Allthough the DPFE sensor should have thrown a CEL.

 

[SHOsFAST]

DPFE's usually cause a rough or stumbling idle and a noticeable lack of power... and dont always throw codes, but I am thinking the FP was probably the problem.

 

[Randall]

Alright, I am back! My computer died after 7 years of service so now I have my new MacBook Pro. anyway, I did change the fuel pump to a Walbro 155 lph unit and there are no more hard starts, and the car took two tanks full of driving to figure out the curves, but now it seems better than ever. There is still intermittent problems with loss of power that usually are accompanied by a rough idle, especially after the car has been warm. I haven't yet changed the IAC valve, but I am leaning towards that. I have a feeling that my intake couplers may also have become to hard to provide a good seal, so they may be causing a slight vacuum leak.
As far as the DPFE, I had a code in the memory when I first bought the SHO last month, but it went away by the week after when I checked codes again. The DPFE tube was totally clogged! I am not sure if non California SHOs have this piece, but basically I think it works with the EGR pressure sensor to control the opening of the EGR valve. SHOsFAST, you hit it right on the nose as I think this cleared up a lot of issues after the tube was cleared out!
If anyone knows of a way to test the IAC valve, please let me know, and thanks for the replies so far!
-Randall

 

[SASHO91]

Two ways to test the IAC....
One is just pull it off the intake manifold and have a friend turn the car on...
you should see it move.... leave it the connetor on though..

The other, is take it off, and run a 9v battery to it with some wire... it will move when the voltage is applied...


EDIT: also, if you just pull the connector when the car is running, and it dies within a few seconds... then its working...

 

[CerberuS]

EGR system is only installed on ATX 93-95 , AND all CA SHOs.

All MTX non CA , doesnt have EGR system.

 

[rigol2000]

DPFE's usually cause a rough or stumbling idle and a noticeable lack of power... and dont always throw codes, but I am thinking the FP was probably the problem.

What's a DPFE? And where is it?

Thanks

 

[SHOspazz92]

What's a DPFE? And where is it?

Thanks

Word, I would like to know what this peice is as well, and do EGR equiped MTX's (California MTX's anyways) Have this Peice?

-Sam

 

[ritmusic2k]

Also interested in learning what 'DFPE' stands for...

Randall,

FYI - that 542 code is stored in nonvolatile memory, which means that it only has to occur once, but it will report every time you check codes until it's actively cleared out by disconnecting the battery. And as I just learned from a recent question of mine... all it takes is stalling the car once and it'll generate that code.

All that aside, glad to hear she's running strong now!

 

[SHOck]

Differential Pressure Feedback EGR sensor.

It's how the ECU knows that the EGR valve is working.

 

[rigol2000]

The reason I was asking about it, is that I just bought a 95 ATX that from what I know has been in California it's whole life. I notice that it has less power than my 94 ATX (stock). Granted, I've done alot of maintenance on my 94. I don't know if there is a way of telling if my 95 was a California car. Did CA cars have different computer programming for emissions? I was thinking that this may be a cause for less power. Once I find my DPFE tube, I'm going to see if it is clogged.

Just taking guesses.

Chris

 

[Randall]

thanks for the info Ritmusic2k!
The DPFE tube is black in color and not that large located along the firewall and it should connect to the DPFE sensor. I believe the DPFE sensor has vaccum lines going to it, so it shouldn't be too hard to find. I also reset the ECU by removing the battery cables for a while and then it seemed to help. Nick Chrimes, a SHO legend and mechanic out here in California told me that the EGR system will rob a small amount of horsepower from the engine by making it run slightly leaner and by making the gas/air mixture dirty by recirculating unburnt fuel and emissions. I don't believe he meant a big difference though, probably only a few horses.