Slight RPM hunting after a warm start

[edwinn]

1997 Riviera - 3.8 Liter, L36 - 135,836 miles


I'm getting some RPM chugging or stumbling after a warm start. The engine turns over quickly as always, but stumbles about three times, then smooths out. About three cycles in three seconds. It just started happening in the last month. Do you think it could be spark plugs (still original), vacuum leak, a sensor?

IOW what would a GM Tech look for? There's no problem with cold starts.

Thanks,


-Ed

[maxwedge]

A obvious misfire can do this, plugs should be replaced usually by 75k if they are Plats. Dirty throttle body, gummed up idle air control, any vacuum leaks, bad fuel regulator allowing fuel to enter the vac. hose, are possibilites. Also sticking egr.

[edwinn]

Originally Posted by maxwedge

A obvious misfire can do this, plugs should be replaced usually by 75k if they are Plats. Dirty throttle body, gummed up idle air control, any vacuum leaks, bad fuel regulator allowing fuel to enter the vac. hose, are possibilities. Also sticking EGR.

1997 Riviera - 139,452 miles


Thanks for that. I'll start with the spark plugs.. already bought a set. Some of the vacuum lines (to fuel rail) are just about 'shot.' Trouble is the right angle 'diameter reducer' hose fittings are hard to find!

The dealer is recommending Induction Service again. It's been 5-1/2 years and 35k miles already since the last IS.

Thanks,


-Ed

[edwinn]

1997 Riviera - 139,754 mi


Low fuel pressure - Some dealer guys looked over the fuel system and engine for a couple days last week on the start-up RPM hunting issue. They didn't find any vacuum hose leaks. I happen to think there are 2 or 3 original Vac tubes and fittings that need to be replaced!! They checked the Fuel Pressure and reported it was LOW measuring 28 psi at start-up (when it should be 48 to 55 psi) then rising to the correct pressure. Sorry don't have the pressure rise time, but it's a matter of seconds.

Would installing a new Fuel Pump help this condition? or waste of time and money?


-Ed

[enslow]

Pull the vacuum line off the fuel pressure regulator. If you smell gasoline, replace it immediately.

Also, if you have a fuel pressure tester of of your own, you can do a complete fuel pressure test. One thing to check is what happens to the fuel pressure when the vacuum is removed from the regulator. It should rise to full pressure immediately.

Sent from my GT-P3113 using Tapatalk 4

[edwinn]

Originally Posted by enslow

Pull the vacuum line off the fuel pressure regulator. If you smell gasoline, replace it immediately. Also, if you have a fuel pressure tester of of your own, you can do a complete fuel pressure test. One thing to check is what happens to the fuel pressure when the vacuum is removed from the regulator. It should rise to full pressure immediately.

1997 Riviera - 139,781 miles


Thanks for that,

The RPM is even more unstable in the first 3-5 sec after a WARM engine start-up. It stumbles and chugs, then smooths out. This is annoying! and it's worrisome. Here's a photo of my Fuel Regulator with Test Port, and the inlet Fuel Line.. which makes a 'U' underneath to the regulator and fuel rail.




On my engine the Return Fuel Line is attached to the back rail where it loops.


Other issues

The dealer guys tested the vacuum pull-down and had no comment whether it was low, leaky or bad. You can see the tiny hard-plastic vac line (dashed YELLOW) going behind, and two BIG reducer elbows (YELLOW arrows) on each end. These are the fittings that are old and dry rotted, and they don't seem to be available anymore.

There's another straight Reducer fitting on the far right (YELLOW arrow) on the Purge tube that's in bad shape!


The BLUE arrow is the inlet Fuel line, and the RED arrows are the Purge tubes. There's a new vacuum solenoid in lower center of photo. The local dealer did a GREAT job with Fuel Service 4 or 5 years ago, installing new FUEL tubes to the engine!




So where to go from here? Buy a fuel pressure gauge? There is no smell of fuel after pulling the vac line off the regulator. Many of these hoses and fitting were known to be deteriorated 3 or 4 years ago. The rubber would come off onto you fingers from handling them.

A new FUEL SENDING UNIT #25319664 SENSOR KI 3.107 $110.64 was installed in July 2001 at 83,751 miles because the Fuel Gauge was erratic.


-Ed

[edwinn]

Originally Posted by maxwedge

A obvious misfire can do this, plugs should be replaced usually by 75k if they are Plats. Dirty throttle body, gummed up idle air control, any vacuum leaks, bad fuel regulator allowing fuel to enter the vac. hose, are possibilities. Also sticking egr.

I sent a link to your reply to the GM service adviser, and they may or may not have checked the FR for leaking diaphragm. A guy over in the TurboBuick forums posted a video on testing the FPR. That may be the ticket. It matches the symptoms.


-Ed

[edwinn]

Video of Fuel Pressure test

A video of yesterday's Fuel Pressure start-up test is pasted below. There are a few more FPT videos.

http://home.comcast.net/~edwinn/97_Riv_FP_startup.mp4 (12.2MB)

What do you think about that? Is it normal for the gauge to `jitter´ like it's doing? Notice the pressure goes LOW right after the engine starts, then is constant, then becomes erratic. Why does the FP rise after the engine is turned off?


Fuel pressure measurements

Here are some pressure readings taken yesterday.

                Fuel Pressure (PSI)

 Test   Ignition  After   FPR Vac   Engine   After
  #       On      Start   Removed    Off     5-min
---------------------------------------------------
  1     30
  2     34
  3     40->38(1)
  4     37->35
  5     43->39
  6     37
  7     37        41-45(2)  47-51     48
  8     46->45    41-45     47-51     43      51(4)
  9     45->44    39-48     47-50     44      51
 10     47->46                        45      51
 11     44->42    33-45(3)            40

Notes:
(1) Initial pressure, drops to ->P after ~1 min.

(2) Pressure jittery, range of gauge fluctuation.

(3) Starts up at 42, drops to 33 and chugs, smooths out to 43, then jitters 41-45.

(4) Fuel pressure rises after Eng turned Off.

Pressures plotted

Since we're technical guys, I went ahead and plotted the measured Fuel Pressures from Sunday and some trends emerge. Let's have a look.



Yellow and Turquoise lines form envelope of gauge/pressure flutter.


1. Over the 11 tests, the Ignition On pressure tended to increase, even though FP was relieved after each run. See dashed BLUE slope line.

2. The VIOLET line is the pressure ~1 min after Ignition On (actually when the air-shock pump started up) and represents the pressure bleed-off rate. It dropped 1-2 psi/min, which isn't very fast I reckon.

3. It wasn't until test #7 that i started the engine, and the YELLOW and TURQUOISE lines are the min/max Idle pressure values. So that's a look at the 'gauge flutter.' Test #10 envelope pressures were faked because no data was recorded.

4. On test #11, an assistance (lovely Ms Carol) helped, so i was able to view the Fuel Pressure immediately after it started.. during the 'start-up instability' period which is what the problem is all about. THAT'S where the recorded Min pressure is lowest... it captured the 'chugging' just after the engine turned over (see video).


Thanks


-Ed

[enslow]

Could you run another fuel pressure test and do the following:

1) with the engine running, pull and plug the vacuum line from the fuel pressure regulator. The fuel pressure should rise immediately.
2) Do a snap throttle test with the the vacuum line attached to the fuel regulator. Fuel pressure should rise instantly as the vacuum drops.

I think you should also check your vacuum from the fuel pressure regulator line, then from a source close to the manifold. I'in beginning to suspect an engine vacuum problem.

Sent from my GT-P3113 using Tapatalk 4

[edwinn]

Originally Posted by enslow

Could you run another fuel pressure test and do the following:

1) with the engine running, pull and plug the vacuum line from the fuel pressure regulator. The fuel pressure should rise immediately.
2) Do a snap throttle test with the the vacuum line attached to the fuel regulator. Fuel pressure should rise instantly as the vacuum drops.

I think you should also check your vacuum from the fuel pressure regulator line, then from a source close to the manifold. I'm beginning to suspect an engine vacuum problem.

Yeah good reply.. as for issue #1, notice the FPR Vac Removed column in the posted table. See how the pressure rises from the After Start column by ~6 psi. So yes the FP rises when Vac line is pulled, but I didn't plug it each time.. only a couple times. Still developing the test and data sheet.


Also you can see the "engine running" pressures (columns 3 & 4) in the table are FLUTTERING, that is the gauge needle is jittery!

See posted video. Is the VIDEO link working for you guys? Can someone please comment on the FP video?

Thanks,


-Ed

[Tech II]

Good video......

Never saw that before......almost looks like a bad pump...hard to imagine a regulator would do that or an injector....

[rkvons]

How's your fuel filter?

[edwinn]

Originally Posted by rkvons

How's your fuel filter?

1997 Riviera - 139,794 miles


The only fuel filter I'm aware of on this vehicle is the one under the LEFT REAR passenger seat pan where the electrical harness exits and goes aft. Service records have the Fuel Filter replaced in Oct 2007 at 100,410 miles. That was 6 years and 39,000+ miles ago.



And now I'm a little confused at what they did on earlier visits. There was BG Induction Service in May 2008, and that was probably when they put in the new fuel tubing under the hood? ..


-Ed

[edwinn]

Originally Posted by Tech II

Good video...... Never saw that before......almost looks like a bad pump...hard to imagine a regulator would do that or an injector....

Hey thanks for the great reply!


-Ed

[edwinn]

1997 Riviera - 3.8 Liter L36 - 139,794 miles


Fuel Pressure Videos

Three more videos processed, and the original video re-converted for better quality.


1. Ignition On (5.4 MB)

http://home.comcast.net/~edwinn/97_R...gnition_on.mp4

2. After vacuum line removed for FPR (4.8 MB) - pressure is 47-51 psi, NOT 43-50 in the audio track!

http://home.comcast.net/~edwinn/97_R...no_FPR_vac.mp4

3. Engine Start (12.2 MB) - demonstrates "Slight RPM Hunting after a Warm Start" but just barely, often worse!

http://home.comcast.net/~edwinn/97_Riv_FP_startup.mp4

4. Engine Off (2.4 MB)

http://home.comcast.net/~edwinn/97_R...ngine_stop.mp4

Will do all these over again now that we're better organized. Last Sunday was just trying out the new gauge and "developing the test."


-Ed