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1994 STS HARD STARTING, SURGING, SMOKING


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1994 STS hard starting at all temperatures. At idle engine will surge from 600 to 1200 RPM. Most of the time the exhaust smell like its too rich and is black and some times exhaust is white but does not smell like oil. Had two codes- P031 MAP SENSOR SHORTED and P095 STALL DETECTED. Both codes were HISTORY not CURRENT. Cleared the codes and Changed the map sensor last night and the idle has stabilized a little and starting is a little easier. Still have black smoke and sometimes white aswell. Ran car for half an hour after map sensor change and no codes present. ANYONE HAVE ANY IDEAS?? THANKS IN ADVANCE FOR YOUR FEEDBACK. OH, one more question - when I took out the old map it did not have a vacuum hose connected to it. It just snapes down into place and gets the electronic 3 prong connector. IS THIS CORRECT?? IT GETS NO VACUUM HOSE??

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Is the MAP sensor inside the manifold on the '94? If so, it is exposed to manifold pressure and would not need a vacuum hose like one mounter out side the manifold. How about the FPR? Have you checked it for leaks?

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Where is the FUEL PRESSURE REGULATOR located on a 1994 STS?? Is it under the top cover that has the 12 torx bolts and what does it look like. What is the test procedure for the FPR and souldn't I be getting a code?? THANKS, CHRIS SCOTTY please ring in on this one as I live less than 10 miles away. My cell # is (516)987-7610

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Mmmmmnnnn.......its rare for MAP to go bad.

There is a TSB about 1994 computers not 'seeing' the MAP correctly. I can post that info if needed.

I would check the fuel pressure regulator first.

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Chris, I am not familiar with the 94 and don't want to steer you wrong. I think the 94 has differences compared to my 96.. My top cover has 4 plastic nuts that hold it down, you say you have 12 torx?

Anyway, the FPR should be attached to your fuel rail or fuel delivery system some place, here is a photo of MY FPR for 1996 but I do not know if it is applicable to your 94.

It is sounding like you are having more than one problem, the black exhaust is normally the result of a rich mixture, so looking at the FPR is a good idea, pull the vacuum hose and see if fuel is present.

Also, do a search on the 94, the phenolic plates need to be cleaned on your year.

*******************************************************

Here are prior posts by bbobynski regarding the phenolic plates:

There is no nead to clean out the manifold itself or even to take the cover off. Just remove the inner 4 bolts that are recessed into the cover and the entire manifold will lift up.

All the gaskets should be reusable. Have some RTV handy so if one of the gaskets is damaged or torn you can patch it locally with a dab of the RTV.

The phenolic plates are all that need to be cleaned. Just scrape the carbon out of the recessed passages with different size screwdrivers and such. Nothing fancy.

The recessed slots in the head surface are also good to clean out and the mouse holes mentioned that are machined into the cylinder heads are very important to clean out. Those are the small machined notches at each intake port.

There is also a hole in the cylinder head at each end that leads towards the rear of the engine that feeds the EGR into the phenolic plates that should be checked. It is usually pretty clean but it should be checked for carbon buildup.

Remember that the idea is to get the majority of the carbon out of the grooves in the phenolic spacer plates. It is going to get sooty again pretty soon so you don't need to be squeeky clean...just clean the majority of the crud out and let'er rip.

**********************************************************

Frequent doses of WOT will exercise the rings and keep the combustion chambers clean....but.....will likely have little or no effect towards cleaning out the phenolic spacer plates... The WOT/decel manuevers are purely to exercise the rings and keep the combustion chambers clean...not the EGR system. The EGR system only operates at part throttle....so...full throttle operation disables it and has no effect on it.

The EGR passages in the phenolic spacer plates eventually clog up with carbon simply due to the EGR flow and the soot from the exhuast gases accumulating over 100K of operation. No way around it...just have to remove them and clean them.

Understand that ONLY the 93/94 engines have the phenolic spacer plates between the intake manifold and the cylinder heads. The 93/94 engines have the large diecast intake manifold "box" with the perimter row of screws in the lid holding the silver top cover on. That is the intake system that has the phenolic spacer plates for the individual cylinder EGR introduction. The 95 and later engines have the one piece injection molded plastic intake with the single point EGR introduction into the intake plenum. The 95 and later engines have the silver top cover that is just a beauty cover...not a functional part like the 93/94 covers were (the top of the intake manifold).

So...WOT will NOT keep the phenolic spacer passages clean...but...you only need to worry about cleaning them if you have a 93 or 94 Northstar.

post-3-1114087443_thumb.jpg

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LOGAN, Please do post the TSB info on the 1994 computer not "seeing" map correctly. I changed the map sensor because of a P031-shorted map signal. Is the FPR located under the intake cover that has the 12 torx bolts - somewhere near the map sensor. My map sensor did not have a vacuum hose connected to it. Is this correct that it just snaps in under the intake cover with no vacuum hose connected. THANKS, CHRIS

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LOGAN, Is the firt picture showing the FPR and how do I test it? What are acceptable PSI's and should I get a code from my diagnostics. THANKS, CHRIS

Chris,

The FPR in your car is under the intake manifold. There isn't a vacuum hose connected to it as the entire cavity is under engine vacuum.

Remove the manifold, turn the key to "on" but DO NOT START THE ENGINE as it will be unthrottled. Look at the FPR nipple - there should not be any gasoline at the nipple. If there is, it needs to be replaced.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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... My map sensor did not have a vacuum hose connected to it. Is this correct that it just snaps in under the intake cover with no vacuum hose connected. THANKS, CHRIS

Yes. On the 94's, the MAP sensor does not have a vacuum hose connected. The MAP sensor is mounted within the intake manifold, so it gets the vacuum signal directly.

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SUBJECT: HARSH CHUGGLE, SURGE, MISS, HESITATION ACCOMPANIED BY A HISTORY DIAGNOSTIC TROUBLE CODE PO71 (CHECK/REPAIR PER DTC)

MODELS: 1993 CADILLAC ALLANTE, ELDORADO COUPE AND SEVILLE SEDAN 1994 CADILLAC ELDORADO, SEVILLE AND CONCOURS WITH 4.6L ENGINE (VINS Y AND 9 - RPOS LD8 AND L37)

CONDITION:

SOME OWNERS MAY EXPERIENCE ENGINE ROUGHNESS, HARSH CHUGGLE DURING HIGHWAY DRIVING, SURGE, MISS AND HESITATION WITH NO MIL OR MESSAGE AND A HISTORY DIAGNOSTIC TROUBLE CODE PO71.

CAUSE:

CODE PO71 SETS WHEN THERE IS A RAPID CHANGE IN MAP WITH NO CORRESPONDING CHANGE IN THE TP SENSOR OR RPM. A DTC PO71 MAY BE A SYMPTOM OF A DRIVEABILITY CONDITION SUCH AS BACKFIRE, STALL, OR LOSS OF POWER. A FALSELY SET DTC PO71 MAY BE CAUSED BY INTERNAL "NOISE" IN SOME NORTHSTAR PCMS INDUCED INTO THE INTERNAL PCM MAP CIRCUIT.

NOTE: ENGINE OIL NEAR OR ON MAP SENSOR IS A NORMAL CONDITION. MAP SENSORS SHOULD NOT BE REPLACED IF ENGINE OIL IS PRESENT.

CORRECTION:

THE NORTHSTAR'S PCM HAS BEEN REDESIGNED TO CORRECT THIS INTERNAL CIRCUIT "NOISE" CONDITION. TO DIAGNOSE AND REPAIR THE ABOVE CONDITIONS, PERFORM THE FOLLOWING PROCEDURES:

1. IF THE VEHICLE HAS MULTIPLE DIAGNOSTIC TROUBLE CODES AND DTC PO71, DIAGNOSE AND REPAIR THE **OTHER** DIAGNOSTIC TROUBLE CODES FIRST.

2. IF THERE WAS A BACKFIRE OR STALL, DIAGNOSE THAT CONDITION AS OUTLINED IN SECTION 6E-B OF THE SERVICE MANUAL.

3. IF THE VEHICLE IS A 1993 AND HAD A LOSS OF POWER AFTER AN EXTENDED HIGHWAY DRIVE, PERFORM THE PROCEDURES OUTLINED IN CORPORATE BULLETIN 216306 (GM OF CANADA 93-6-118; CADILLAC T-93-49 IN GROUP 6E).

4. IF THE VEHICLE IS A 1993, PERFORM THE PROCEDURES OUTLINED IN CORPORATE BULLETIN 316005 (GM OF CANADA 93-6-118; CADILLAC T-93-101 IN GROUP 6E).

5. IF THE VEHICLE HAS A HARSH CHUGGLE DURING HIGHWAY DRIVING, SURGE, MISS OR HESITATION ACCOMPANIED BY A HISTORY DTC PO71, INSPECT THE SERVICE PART NUMBER ON THE PCM. IF IT IS ONE OF THE PRESENT SERVICE PART NUMBERS LISTED BELOW, REPLACE THE PCM USING ONE OF THE APPROPRIATE **NEW** SERVICE PART NUMBERS LISTED BELOW.

6. IF THE PCM HAS THE NEW SERVICE PART NUMBER, FOLLOW THE APPROPRIATE SERVICE INFORMATION MANUAL PROCEDURES FOR DTC PO71 AND INSPECT THE MAP SENSOR CONNECTIONS AND CIRCUITRY.

PARTS INFORMATION:

PRESENT NEW DTC PO71 DESCRIPTION APPLICATIONS SERVICE P/N SERVICE P/N ----------- ------------ ----------- ------------- POWERTRAIN CONTROL MODULE 1993 V WITHOUT V4J 16159277 16207768 POWERTRAIN CONTROL MODULE 1993 V,E,K WITH V4J 16180517 16197429 POWERTRAIN CONTROL MODULE 1994 E/K + KSP 16196402 16197429

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SUBJECT: INTERMITTENT MISS, SURGE, OR LACK OF POWER WHILE DRIVING USUALLY ON ACCELERATION (REPAIR WIRING/REPOSITION HARNESS)

MODELS: 1993-94 CADILLAC ELDORADO, SEVILLE, AND CONCOURS WITH 4.6L NORTHSTAR ENGINE (VINS Y, 9 - RPOS LD8, L37)

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * THIS BULLETIN CANCELS AND SUPERSEDES BULLETIN NUMBER 476401 TO * * CORRECT THE CATEGORY IDENTIFIER/GROUP REFERENCE. PLEASE DISCARD * * BULLETIN 476401 (GROUP REFERENCE 6 - ENGINE). * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

CONDITION:

SOME OWNERS MAY EXPERIENCE AN INTERMITTENT MISS, SURGE OR LACK OF POWER ON A MODERATE TO HARD ACCELERATION OR AT STEADY SPEEDS WITH NO PCM DTC CODES SET.

CAUSE:

THIS CONDITION MAY BE CAUSED BY AN INTERMITTENT SHORT TO GROUND ON THE PARK/NEUTRAL INPUT (PI79) TO THE PCM. THE PARK/NEUTRAL INPUT (PI79), IS A NORMALLY "HI" INPUT TO THE PCM WHEN THE GEAR SELECTOR IS IN ANY OF THE DRIVE POSITIONS AND "LO" IN PARK OR NEUTRAL. IF (PI79) GOES "LO" WHILE DRIVING, THE PCM WILL ENABLE THE TORQUE MANAGEMENT MODE. DURING THE TORQUE MANAGEMENT MODE, THE PCM WILL REDUCE ENGINE POWER BY RETARDING THE TIMING AND SHUTTING OFF 4 INJECTORS. THE PARK/NEUTRAL INPUT, CIRCUIT 434 (AN ORANGE/BLACK WIRE), MAY BE SHORTING TO THE TRANSAXLE CASE NEAR THE PRNDL SWITCH CONNECTOR (SEE FIGURE 1).

CORRECTION:

TO REPAIR THIS CONDITION, USE THE WIRING REPAIR PROCEDURES IN THE SERVICE MANUAL, SECTION 8A-5. AFTER THE CIRCUIT HAS BEEN REPAIRED, THE HARNESS SHOULD BE REPOSITIONED USING PLASTIC TIE STRAPS TO PREVENT CONTACT WITH THE TRANSAXLE CASE CAUSED BY ENGINE MOVEMENT DURING ACCELERATION.

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SUBJECT: MALFUNCTION INDICATOR LAMP (SERVICE ENGINE SOON) ON AND/OR LOSS OF ENGINE POWER (INSPECT/REROUTE ENGINE HARNESS)

MODELS: 1994 CADILLAC CONCOURS (BEFORE VIN BREAKPOINT 266253) ELDORADO (BEFORE VIN BREAKPOINT 615542) SEVILLE (BEFORE VIN BREAKPOINT 829859)

CONDITION:

SOME OWNERS MAY COMMENT ON MALFUNCTION INDICATOR LAMP (SERVICE ENGINE SOON) ON AND/OR LOSS OF ENGINE POWER.

CAUSE:

THE NORTHSTAR ENGINE HARNESS MAY HAVE BEEN ROUTED OVER THE ENGINE/TRANSMISSION MOUNTING BRACKET IN THE AREA NEAR THE POWER STEERING GEAR. THIS ROUTING PERMITS THE HARNESS TO CONTACT THE EXHAUST SYSTEM, WHICH COULD EVENTUALLY WEAR THROUGH THE WIRING INSULATION.

CONTACT OF THE NORTHSTAR ENGINE HARNESS WITH THE EXHAUST MANIFOLD COULD CAUSE ONE OR MORE OF THE FOLLOWING CONDITIONS:

1. MALFUNCTION INDICATOR LAMP (SERVICE ENGINE SOON) ON, ALONG WITH ONE OR MORE DIAGNOSTIC CODES SET.

2. LOSS OF ENGINE POWER, A RESULT OF AN OPEN ENGINE GROUND G102.

CORRECTION:

DETERMINE IF THE ENGINE HARNESS ROUTING IS CORRECT AND REROUTE HARNESS AS FOLLOWS:

1. DISCONNECT NEGATIVE BATTERY CABLE.

2. POSITION VEHICLE SO THAT THE LOWER ENGINE AREA BEHIND THE RIGHT FRONT WHEEL IS ACCESSIBLE. NO DISASSEMBLY IS REQUIRED TO GAIN THIS ACCESS.

3. EXAMINE THE LOWER FRONT CORNER OF THE ENGINE NEAR THE TRANSMISSION MOUNT ASSEMBLY. IN THIS AREA, A BRANCH OF THE ENGINE HARNESS ROUTES BELOW THE HEATER INLET/OUTLET PIPES. FROM THERE, IT PROCEEDS DOWNWARD AND SPLITS INTO FOUR COMPONENT BRANCHES. THESE BRANCHES GO TO: (1) ENGINE GROUND G102, (2) KNOCK SENSOR, (3) VEHICLE SPEED SENSOR, AND (4) POWER STEERING PRESSURE SWITCH. CHECK THAT THESE BRANCHES ARE ROUTED AS SHOWN IN FIGURES 1, 2 AND 3. THE HARNESS AND ALL FOUR BRANCHES OF THE HARNESS SHOULD BE BELOW THE TRANSMISSION MOUNTING BRACKET, WITH THE POWER STEERING PRESSURE SWITCH BRANCH THREADED THROUGH THE TRANSMISSION MOUNT BRACKET.

4. IF THE ENGINE HARNESS WAS INSTALLED IN ANY MANNER OTHER THAN THE WAY SHOWN IN STEP 3, INSPECT THE HARNESS FOR DAMAGE.

5. IF THE ENGINE HARNESS IS DAMAGED, REPAIR THE WIRING AS SPECIFIED IN SECTION 8A-5 OF THE APPROPRIATE SERVICE MANUAL.

6. IF THE ENGINE HARNESS ROUTING DIFFERS FROM THE CONFIGURATION DESCRIBED IN STEP 3, REROUTE THE HARNESS AS FOLLOWS.

7. UNPLUG THE POWER STEERING PRESSURE SWITCH CONNECTOR, KNOCK SENSOR CONNECTOR AND VEHICLE SPEED CONNECTOR.

8. DISCONNECT THE ENGINE GROUND G102 LOCATED NEAR THE ENGINE KNOCK SENSOR BY REMOVING ONE BOLT.

9. REROUTE THE ENGINE HARNESS AS DESCRIBED IN STEP 3 ABOVE (FIGURES 1, 2 AND 3).

10. INSTALL ENGINE GROUND AND SECURE TO ENGINE USING BOLT. TIGHTEN BOLT TO 11 N.M (8 LBS. FT.).

11. CONNECT THE VEHICLE SPEED SENSOR CONNECTOR, KNOCK SENSOR CONNECTOR AND POWER STEERING PRESSURE SWITCH CONNECTOR.

12. RECONNECT NEGATIVE BATTERY CABLE.

I posted 3 bulletins. I would think the PCM issue would have been found years ago.

Logan Diagnostic

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  • 2 months later...

I have the harsh chuggle problem at highway speeds. The car also misses at idle, and at all speeds but is most annoying on the highway. The part number on my PCM is 16180517. I am unclear after reading the service bulletin which groups of numbers in the bulletin are for the faulty PCM and which are for the new PCM.

Logan, would you clarify this?

I want to determine if I need to replace this PCM.

Thanks,

STSdriver

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That is ....technically a fairly old service bulletin. I would suspect that the part #s listed have long been updated several times. So I would ignore the actual service part # issue.

What I would recommend, is drop down the PCM and tap test it while the car is running. Any burps or stalls or other funny business while tapping indicates a bad PCM.

Sometimes, the PCM is ok until it has a chance to warm up, maybe 10 minutes or so.

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  • 6 months later...

Hello Logan,

I am having similar surging problems with my '94 STS. Lately it has gotten so bad that when it acts up, the car will stall and then start hard or not at all. I have it at the shop; they have tracked a 5 volt reference through the TP sensor and MAP sensor and a ground and suspect the PCM is the problem. Both sensors have been recently replaced. They suggest I take it to a dealer for further diagnostic before I replace it. Do you think that the "tap" test would replicate my Caddy's troubles if the PCM is bad? Is this a part I can get at GMPARTSDIRECT or elsewhere and install myself?

Thanks for your help Logan,

Eric

That is ....technically a fairly old service bulletin. I would suspect that the part #s listed have long been updated several times. So I would ignore the actual service part # issue.

What I would recommend, is drop down the PCM and tap test it while the car is running. Any burps or stalls or other funny business while tapping indicates a bad PCM.

Sometimes, the PCM is ok until it has a chance to warm up, maybe 10 minutes or so.

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Where is the FUEL PRESSURE REGULATOR located on a 1994 STS?? Is it under the top cover that has the 12 torx bolts and what does it look like. What is the test procedure for the FPR and souldn't I be getting a code?? THANKS, CHRIS SCOTTY please ring in on this one as I live less than 10 miles away. My cell # is (516)987-7610

KHE hit it on the nose, the FPR is under the top cover. It is located on the drivers side toward the back. It is round and clicks into the fuel rail holder with a nipple on top and has a fuel line attached to it on the bottom that just snaps in place. Turn the key on(engine off) to pressurize the fuel system. If it is bad it will spew gas like a whale. Do not get an aftermarket FPR, I got one at Advance and it did not last one year. The one I got from NAPA would not fit in the holder. Get a GM one. No codes will set with a faulty FPR on a 94. Make sure you clean all the excess gas out of the intake before reinstalling the cover. DO NOT START THE CAR WITH THE COVER OFF. It will rev up full throttled and harm the engine. If the FPR does not leak, pull the injectors out and see if one is leaking. Once again pressurize the fuel system and no injector should be leaking.

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A rich fuel mixture can be commonly from a leaking Fuel Pressure Regulator. This condition can be hard on the 02 sensors that may also not be working correctly.

Safety First:

To access the FPR you will need to remove the intake cover.

Never start a 93-94 Northstar with the top of the intake removed!

With the top of the intake removed you have no throttle control. The engine will run wide open and may rapidly destroy itself or cause serious engine damage. The top of the 93-94 intake manifold is not a decorative piece to just take off, as in later models. It is the "cap" of the intake plenum. Removing and replacing the top unnecessarily will start to compromise the gasket of the Fuel Rail and will lead to poor sealing, idle and drivability problems.

The oily residue in the bottom of the intake is normal and it is not necessary to sop it up; it will just come back. It is from crankcase and gas vapors settling out.

The Torx screws on the 93-93 intakes are special; in a strong backfire it will allow the intake cover to lift slightly against the rubber isolator pieces on the screws, to relieve pressure, rather than burst the intake manifold. If you lose one replace it with the correct screw and isolator combination from the dealer.

Unless you are really inspecting for a fuel injector or FPR problem, you should not remove the 93-94 intake cover.

To check the FPR:

Unplug the (4) coil wire plugs at both ends of the coil pack for safety. If the engine starts with the cover off it will runaway; due to uncontrolled air intake.

Pull off the front spark plug boots.

Lift the plastic sparkplug wire carrier from the intake and shift out of the way.

Remove the magnesium intake cover (all of the Torx screws, not the 4 hex head screws).

Turn the key to the ON position and look for seepage from the nipple, on top of the FPR.

Any fuel coming out of the FPR nipple, and it needs to be replaced.

To replace the FPR you will need to lift the fuel rail out of the way. Exercise care or the fuel rail will break.

Replace plastic sparkplug wire carrier, and plug the spark plug boots back in.

After replacing the intake cover be sure to plug the (4) coil wire harness plugs back in.

To check the 02 sensors:

Start the engine and allow to completely warm up.

Enter the diagnostics mode and review the data for the 02 sensors. The voltage readout should vary from 0-1.16 volts. The value should vary frequently; a slow or not changing value means that the sensor is not working correctly.

PD32 = Front 02 sensor

PD33 = Rear 02 sensor

Front sensor is easy to replace, rear is hard.

I have also placed a .PDF file on my web storage site, that has the service manual pages, with illustrations and parts list, to help guide you.

Cadillac STS Information

Scroll down to Fuel Pump Regulator.

Drive'em like you own 'em. - ....................04 DTS............................

DTS_Signature.jpg

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  • 5 years later...
  • 1 year later...

Reopening an old thread...any 94's? FPR problems

About to find out. I failed emissions here in Colorado with my '94 Seville STS. HC failed but CO and NOX were borderline. Replaced all plugs, ignition coils, and wires. Cat also recently replaced. Still failed. Exhaust has always smelled strong of gas IMO. Check codes and P032 and P071 for MAP present. Removed intake manifold cover and all I can say....darn. Everything inside is covered in fuel/oil (hard to tell if what looks like oil isn't just old gas that has mixed with the plastics inside the manifold). The whole harness, injectors, MAP sensor, FPR, even gas sitting in bottom of intake. Suspecting FPR at this point. Maybe leaking injector. The place where the injector wiring harness goes down through the plastic fuel rail mount was filled up with fuel/oil, like all the wires were inundated in it, as in, imagine a cup filled up. Wondering if that could cause a short.

I checked the intake manifold cover gaskets, seem to be ok. Since MAP and FPR are inside intake, there are no vacuum lines to them. Wondering if there are any other potential areas where a vacuum leak could affect the intake, as I would suspect that this would act the same as a vacuum leak on the line directly to the FPR on most vehicles, and cause a rise in fuel pressure. At any rate, I suspect that a vacuum leak of that nature would not cause all the fuel that I see in the intake area. Replacing FPR, MAP, all O-rings (fuel rail, injectors), and sending all injectors for cleaning/testing. Will update when repairs complete.

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