sort of stumble at idle

[JimBobC4]

Tom94,

I have been watching this thread a while and see you have tried everything else. So here's my theory.

I think from what your describing you have a vacuum leak. Internal or external. It sounds like your getting air into the intake from an outside source.

If you give it a quick but spirited blip on the throttle ( in park at idle )
does the engine bog a little end then catch up so to speak ? Like the skip goes away after about 1k rpms ? a quick hard stab makes it bog but it slowly revs up fine ?

Maybe you have a bad intake gasket ?
The intakes are notorious for leaking.
Check the torque on the center bolts of the intake and see if those puppies spin out like they were hand tightened.

If so I would put some intake gaskets on it.

just a thought !

 

[garyg]

Won't hold an idle

Thnx Tom94,

I'll try that today...bliping the throttle. The only error code readout I had previously was a 54 indicating fuel pump relay. I replaced it and in error said the fuel pump regulator in my last reply. Now I get a clean 12 reading but the start problem persists. My 1988 vette has 50,000 original miles and has never had any mechanical problems ( only routine replacements, plugs, battery, catalytic converter, mufflers, etc. ). This car is covered and garaged and as they say... you can "eat off the engine".

I will search for the "air leak"... I hope you 'r right.

 

[Toms007]

Tom94,

I have been watching this thread a while and see you have tried everything else. So here's my theory.

I think from what your describing you have a vacuum leak. Internal or external. It sounds like your getting air into the intake from an outside source.

If you give it a quick but spirited blip on the throttle ( in park at idle )
does the engine bog a little end then catch up so to speak ? Like the skip goes away after about 1k rpms ? a quick hard stab makes it bog but it slowly revs up fine ?

Maybe you have a bad intake gasket ?
The intakes are notorious for leaking.
Check the torque on the center bolts of the intake and see if those puppies spin out like they were hand tightened.

If so I would put some intake gaskets on it.

just a thought !

Sorry guys for not responding....I have been watching 6 of my 7 grandkids this week and they are keeping me busy.

JimBob, I am not counting out a vacuum leak, but have not been able to locate one. Yes it hesitates when you hit the gas, but it doesn't smooth out at any point. Also I'm not sure that a vacuum leak (intake gaskets) would just "boom" happen like I have observed on my car.

My next question is where is the fuel pressure regulator on this thing. I can't find any reference to it in the FSM (unless I'm blind...and that could be the case ). LT4Man has suggested that I am still not getting the fuel pressure I need, and that thought is reinforced by GMJunkie. I am thinking the FPR is not functioning properly and I could stand to check that out. I had to replace the FPR on my 99 Silverado, it was a piece of cake, but where is the FPR on the LT1?

Again, thanks to all who have given suggestions or moral support.

 

[garyg]

fuel pressure regulator location

I have Probst " CORVETTE FUEL INJECTION " book and according to it the FPR is located on an LT1, LT4 at the end of the fuel rail assembly at the rear. On a '97-98 LS1 it is between the fuel rails and on '99-01 LS1 it is integral in the fuel filter.

 

[Toms007]

I have Probst " CORVETTE FUEL INJECTION " book and according to it the FPR is located on an LT1, LT4 at the end of the fuel rail assembly at the rear. On a '97-98 LS1 it is between the fuel rails and on '99-01 LS1 it is integral in the fuel filter.

Duh, I have that book too....I just forgot that I had it. Thanks for the heads up...and for jarring my memory.:duh

 

[LT4man]

Take off the plastic fuel rail cover on the driver's side.

At the rear you will see a small metal dashpot which is mounted to the driver's side fuel rail. Remove the vacuum hose from the regulator and check for fuel inside the vacuum hose.

Any fuel in the hose at all, you must replace the regulator.

Make sure you bleed the fuel pressure off before removing the regulator. When you install the new one, be careful of the o-ring.

SAVE the gosh darn

 

[Hib Halverson]

A couple of observations, here.
First, the Probst "Corvette Fuel Injection" book...beware, it's full of errors. See the review at: http://www.corvetteactioncenter.com/reviews/showproduct.php/product/42/cat/27

Secondly, while placing a screen upstream of the EGR valve to prevent pieces of carbon, or other debris carried by the exhaust, from lodging between the EGR valve pintle and seat, might seem like a good idea, beware...

The ECM cal is programmed with a certain amount of EGR flow in mind. You put one of those water faucet screens in the line and you may restrict EGR flow enough that the engine could begin to run like the EGR was partially restricted. Typically, when an engine with EGR suffers no or low exhaust recirc. flow, you get detonation at part throttle and possible drivability issues.

Third, after reading the first post several times, I believe what you think is a "stumble" is really an intermittent misfire at idle. Perhaps you ought to troubleshoot it that way. EGR "hanging open" could be a problem but it's unlikely because you've cleaned it and put a screen in the line but the symptom is unchanged. I'd remove the screen.

Connect your tester, look at the data stream and see what the fuel trims are doing when the misfire occurs.

How does this engine run at high rpm?

Also, "LT4 Man's" mention of the DTC43 as a history code is important. 43 sets for one or more of three reasons: one sensor circuit open, both sensor circuits open or both sensors shorted or grounded. That this is a history code and not a current code implies the issue might be intermittent and intermittents are often problems with wires and connections. Even if this code is not current, I'd check all the KS wires and connections.

Before you start buying injectors or pumps I'd run the fuel pressure tests in the Factory Service Manual. An internet forum thread, while useful information, is not a substitute for proper diagnosis per the FSM. As a fuel supply system issue is probably not the cause of this misfire, I would not replace a fuel pump unless the FSM diagnostics tell you, specifically, that is a solution. If the fuel filter has not been replaced in the last 36,000 miles or three years, I'd replace it.

If you do need a pump, while GM sells the entire tank pump/sender assembly there are vendors, Airtex and others, which sell just the pump. That said, when you buy just the pump, you have to change it and sometimes that requires soldering.

 

[Toms007]

Hib, thank you for your response. You may well be correct in saying the original issue might have been a misfire, as I later found the #8 spark plug to be only finger tight (that might have caused the misfire....yes?).

I can certainly change the EGR back to "pre-screen" configuration....I will give that serious consideration.

High RPM? Not well at any RPM. I'll connect the tester and try to get those readings on the fuel trims. When I look at them they don't mean much to me, but with some expert help they might point the finger at the culprit.

I already replaced the injectors way before this issue surfaced. I guess I jumped the gun on the fuel pump, I bought a new pump from a Chevy dealer and have it installed with still the same issue, No soldering needed to change it out ($$$ not well spent I guess). The fuel filter I changed Monday at the same time as I did the pump (relatively cheap preventative maintenance...in my book)

Hib, things are pretty hectic here and I'm not sure when I will be able to run those scans, but I will post back with my findings. Thank you.

 

[JimBobC4]

My only problem with the fuel pressure theory is that it has 41 psi at idle and still skips. Never mind the running portion.
When at idle the the psi is constant, tells me the FPR is doing its thing, at idle anyway.

I think 41 psi at idle is enough to keep that thing running smooth. My 1995 is at 41 right now. Smooth as silk. Also, if it were a fuel pressure issue it would tend to get worse at higher RPM I would think. More air = more fuel needed here. So it would get worse the higher the RPM's and the temps would rise faster with RPMS, until the engine finally noses over on its itself from a total lack of fuel.

" Also I'm not sure that a vacuum leak (intake gaskets) would just "boom" happen like I have observed on my car. "

Sure it can. With a loose and near failing gasket in a high vacuum situation, it can suck a particle of that gasket right off. Creating an instant hole. Bam like that.
how much vacuum does a SBC have per square inch ? 15 or 20 ?

Also, a vacuum leak or any failure has to start at some point.
Failure of a part is usually a definitive point in time.

if the above items ( FPR strainer etc .) don't work

Try pulling the middle two spark plugs on either side and see if one is more white than the other or at least check the outside of the manifolds for a cylinder that looks like it has been running hotter.

I think you have one cylinder running lean and hot and thats whats making the temp rise and the engine ping.

However, its a guess and free advice, and we all know what thats worth !

I don't want to confuse things any worse. I know you have many opinions on this board of what it is.

 

[Hib Halverson]

Hib, thank you for your response. You may well be correct in saying the original issue might have been a misfire, as I later found the #8 spark plug to be only finger tight (that might have caused the misfire....yes?).

I can certainly change the EGR back to "pre-screen" configuration....I will give that serious consideration.

High RPM? Not well at any RPM. (snip)

Ok. I was confused. I thought this was a misfire at idle that went away as the engine rpm went up.

Yes, if you found one spark plug loose, that might be a misfire.

If tightening #8 doesn't fix it and the miss exists all the time, but not codes are set, along with looking at the fuel system, I'd widen my "net" to engine mechanical problems.

Have you run a compression test?

Inspected the valve springs for a broken spring?

Actually, widening the "net" is actually going to make this thread less useful. What we need is more information about the symptoms gained by "ruling out" some possiblities because, at this point, there are so many potential causes.

 

[Toms007]

Again...Thanks to all who have responded, both here and by PM. There is no way I am going to be able to look at this thing until next week. We leave here within an hour for KC to return three of the granddaughters to their home and parents, so we won't be back until Monday evening some time. I will make a concerted effort to do some of the checking on things on Tuesday. It would certainly be great to find something and get it fixed ASAP. I actually think that this issue (and taking care of 6 grandkids this week) has caused me a case of the shingles (confirmed by Dr this morning).

 

[Toms007]

OK, it's not Tuesday but I did finally get a scan.

Today I got up and read, thoroughly, the FSM. I pulled the valve covers to check for broken springs (none). Then I took my scan tool and checked for set codes (none).

I fired the engine up and the first thing that happened was a "pop" through the TB. Then the engine fired right up. In open loop, at idle it appears to be running "near normal", but as it warms up and goes into closed loop the idle becomes "labored". Put it in gear and it is obvious that there is a miss. SES light is on at this point (see below, 43 Electronic Spark Control Failure).

Here is the data from the scan

Vehicle Scan Report
Data Downloaded from Scanner: 08/10/2008 10:54 AM

Trouble Codes
43 Electronic Spark Control failure
Pending Codes <none found>

1 Desired Idle 600.00 RPM
2 Engine RPM 550.00 RPM
3 Coolant Temp 194.90 °F
4 Oil Temp 142.20 °F
5 Manifold Air Tmp 92.30 °F
6 A/C Pressure 156.00 PSI
7 MAP Sensor 2.23 Volts
8 Throttle Sensor 0.58 Volts
9 Throttle Angle 0.00 %
10 Battery Voltage 12.4 Volts
11 Barometric Press 4.35 Volts
12 Left O2 Sensor 1007.00 mVolts
13 Right O2 Sensor 906.00 mVolts
14 Block Learn Cell 16.00
15 Left Block Learn value 140.00
16 Right Block Learn value 153.00
17 Left Integrator value 128.00
18 Right Integrator value 128.00
19 Left Inj Pulse 6.60 mS
20 Injector Pulse 7.20 mS
21 Mass Air Flow 8.00 gr/sec
22 CCP Duty Cycle 0.00 %
23 Idle Air Mtr Pos 53.00 steps
24 Learned Idle Pos 34.00 steps
25 Spark Advance 14.00 °
26 Knock Retard 7.00 °
27 Knock Sensor 22.00
28 EGR Duty Cycle 0.00 %
29 Vehicle Speed 0.00 MPH
30 Engine Run Time 791.00 seconds
31 A/C Clutch Enabled
32 A/C Requested
33 Park/Neutral Not Detected
34 Cruise Control Not Engaged
35 Brake Depressed Yes
36 TCC Status Not Locked
37 Fan Number 1 On
38 Fan Number 2 On
39 Air Switch Solenoid Off
40 Loop Status Closed
41 Oxygen Sensor Ready
42 R Rear Oxy Sense Ready
43 Learn Control Disabled


Does any of this help with my problem? I am going to go back and study the FSM some more.

 

[Toms007]

OK I am trying to run down the Code 43. FSM says to check Knock Sensors there are two, one on each side of the block.....but where are they? I found what could be one on the passenger side (near the starter behind a shield)?

I can't see anything from the under side on the drivers side of the engine....where is it?

 

[Toms007]

Alright, I found both sensors and did the following checks:

1. DCV with ignition on; both lines were 4.83 volts (specs say around 5 volts, so I am going on the assumption we are close enough).

2. Check resistance of sensors; right bank 3900, left bank 5090 (specs say between 3300 and 4500, so I am going on the assumption that the left bank Knock sensor is bad and needs to be replaced..........Yes or No?)

 

[LT4man]

Alright, I found both sensors and did the following checks:

1. DCV with ignition on; both lines were 4.83 volts (specs say around 5 volts, so I am going on the assumption we are close enough).

2. Check resistance of sensors; right bank 3900, left bank 5090 (specs say between 3300 and 4500, so I am going on the assumption that the left bank Knock sensor is bad and needs to be replaced..........Yes or No?)

Yes, that is correct, Tom.

This makes sense. The left bank knock sensor is pulling timing (retarding the timing) out of the ignition at idle & as you are accelerating.

The knock sensor is mounted in a coolant jacket. You will have to drain your coolant to replace it. Even if you try to do the replacement quickly (w/o draining) you will end up will quite a lot of coolant on your garage floor.

In fact, to flush the LT1/4 engines, the proper way is to remove both knock sensors.

SAVE the gosh darn

 

[Toms007]

New knock sensor ordered. I'll get it changed out and see if that will make any difference.

 

[Toms007]

Knock sensor changed out......No change on the rough running. Did another data stream. Here it is:

Vehicle Scan Report
Data Downloaded from Scanner: 08/12/2008 07:28 PM
Trouble Codes
43 Electronic Spark Control failure
Pending Codes <none found>

Operational Data
1 Desired Idle 712.00 RPM
2 Engine RPM 650.00 RPM
3 Coolant Temp 150.30 °F
4 Oil Temp 92.30 °F
5 Manifold Air Tmp 86.90 °F
6 A/C Pressure 139.00 PSI
7 MAP Sensor 1.70 Volts
8 Throttle Sensor 0.58 Volts
9 Throttle Angle 0.00 %
10 Battery Voltage 13.60 Volts
11 Barometric Press 4.35 Volts
12 Left O2 Sensor 1003.00 mVolts
13 Right O2 Sensor 862.00 mVolts
14 Block Learn Cell 16.00
15 Left Block Learn value 140.00
16 Right Block Learn value 153.00
17 Left Integrator value 128.00
18 Right Integrator value 128.00
19 Left Inj Pulse 5.80 mS
20 Injector Pulse 6.30 mS
21 Mass Air Flow 8.00 gr/sec
22 CCP Duty Cycle 0.00 %
23 Idle Air Mtr Pos 51.00 steps
24 Learned Idle Pos 34.00 steps
25 Spark Advance 20.00 °
26 Knock Retard 1.00 °
27 Knock Sensor 22.00
28 EGR Duty Cycle 0.00 %
29 Vehicle Speed 0.00 MPH
30 Engine Run Time 300.00 seconds
31 A/C Clutch Enabled
32 A/C Requested
33 Park/Neutral Detected
34 Cruise Control Not Engaged
35 Brake Depressed No
36 TCC Status Not Locked
37 Fan Number 1 On
38 Fan Number 2 On
39 Air Switch Solenoid Off
40 Loop Status Closed
41 Oxygen Sensor Ready
42 R Rear Oxy Sense Ready
43 Learn Control Disabled


Major differences; battery Voltage today 13.6 Sunday 12.5
O2 sensors Today Left a little higher Right a little lower
Injector pulse both sides dropped a little
Spark advance today 17-19 Sunday 13-15
Knock Retard today 1 Sunday 6-7
Knock Sensor today's number is double what it was Sunday 44 22

Still have the code 43 even after clearing the codes, it came right back.

 

[LT4man]

Tom, we need to establish a base line.

Do you have a vacuum gauge? Please connect it to intake manifold vacuum. How much do you have when the engine is first started? How much do you have when it is in closed loop after warming up? Is it steady at all times? If it fluctuates, how much does it move?

How much fuel pressure do you have cold/hot? Did you check the vacuum hose at the fuel pressure regulator?

Does it accelerate in neutral better than when in drive?

Have you checked all your ignition connections? Ignition coil connections. Optispark connections. Ignition wire connections. They all must be clean and tight.

Has there been any improvement since installing fuel pump & knock sensor?

As soon as you post your findings, I will try to extrapolate your information.

WILL SOMEBODY PUHLEASE SAVE the

 

[Toms007]

Tom, we need to establish a base line.

Do you have a vacuum gauge? Please connect it to intake manifold vacuum. How much do you have when the engine is first started? How much do you have when it is in closed loop after warming up? Is it steady at all times? If it fluctuates, how much does it move?

How much fuel pressure do you have cold/hot? Did you check the vacuum hose at the fuel pressure regulator?

Does it accelerate in neutral better than when in drive?

Have you checked all your ignition connections? Ignition coil connections. Optispark connections. Ignition wire connections. They all must be clean and tight.

Has there been any improvement since installing fuel pump & knock sensor?

As soon as you post your findings, I will try to extrapolate your information.

WILL SOMEBODY PUHLEASE SAVE the

Gregory, I do not have a vacuum gauge, but I'll get one from school and do the check. I have checked all the vacuum lines and it appears that they are all good. Tonight when coming off idle there was hesitation while in park, I didn't take it off the lift so I can't tell you how it was in drive (other than I put it in gear with the brake on to see how it would idle...rough).

Junkie PMed me and said the same thing about the ignition connections, I'll check those tomorrow. Has there been any change since the FP/KS? Maybe some, but not a significant amount. As I answered Junkie, my oil leak has worsened since the problem....somewhere on the front passenger side (not the intake...I suspect the timing cover), I am now wondering if the oil is spraying/being flung up on some of these ignition components. So I will check that out as I said earlier.

As the time goes by on this issue, I am thinking seriously about pulling the engine this winter and tearing it down (intake, heads, pan, front cover, timing chain, etc) to replace gaskets and freshen it up a bit (maybe rod and main bearings).

I'm getting really frustrated. Oh, BTW....thanks for sticking in here with me......

 

[LT4man]

Tom, do not let a silly old machine make you lose sleep at nite! There are more important things in this life of ours to be really worrying about.

A vacuum gauge is a mechanic's/technician's BEST friend. I still have the one I bought when I was 20 years old. Had it for almost 35 years! The old girl still tells me the truth every time I hook it up.

You might want to "TEE" it into a line on the driver's side of the intake manifold. That way every thing still gets the proper vacuum while you are looking at the gauge.

Any time I get a "driveability issue, I reach for the vacuum gauge. I can tell you how good the compression is, how good the rings are, if any valve springs are broken, if there are any intake manifold leaks and how good the spark plugs & wires are just by looking at that gauge!

Get some sleep & I will see you in the morning, my friend!

SAVE THE