Why the miss ?

[roger longman]

Hi everyone and many thanks for all your help, Finally think I may have nailed it ! Plugged up the EGR hose and idle is smooth, general running is smooth and no miss at low rpm in OD. Now I wonder if someone can tell me the pro's and con's of : opening up the air filter box, taking out the 2 mesh screens from the MAFS and bipassing the heating on the TB,(will try to find a past thread to see how that is done).Should I start a new thread with this ? Roger.

 

[vetteboy86]

Congrats!!! Glad you finally figured it out.

As far as the air box, many have done it. I don't think anyone could argue it hurts. I'm sure there are dyno numbers somewhere. I don't know off the top of my head what the gains are. I have the Vortex ram air system for my car. It was a pretty straight forward install.

On a stock car, or any car for that matter, I don't see how de-screening the maf results in serious HP. Some also say that removing the screens, requires modification of the Maf tables within the bin file for the ECM. I'm not sure I would go that far, but perhaps.

Bypassing the TB, if you are in a warm climate, there isn't any problems doing it. Once again, not sure if measurable HP will equate to the mod, but it is something you can do with you car, that give your car more worth to you. Bonding time if you would!!

 

[Schrade]

Hi everyone and many thanks for all your help, Finally think I may have nailed it ! Plugged up the EGR hose and idle is smooth, general running is smooth and no miss at low rpm in OD. Now I wonder if someone can tell me the pro's and con's of : opening up the air filter box, taking out the 2 mesh screens from the MAFS and bipassing the heating on the TB,(will try to find a past thread to see how that is done). Roger.

Wow - strange. EGR problem will usually cause a vacuum leak and loping idle.

I hate those 'accidental' fixes, cause ya' don't learn anything. But like they say in hopscotch, a lucky hand beats skill every time uh HUH. (or is it horseshoes? skydiving?) :confused

 

[roger longman]

Hi all, Can anyone tell me how to bypass the throttle body, preferably with a picture ? I've looked at past posts but the pics show a later model than mine. Also it would be interesting to know the improvement gained, if any. Roger.

 

[vetteboy86]

The improvement is that any heat that is absorbed from the coolant through the TB and then into the plenum is reduced. Maybe 1-2 hp!

To bypass the TB, locate the two coolant lines, one is flowing in, the other flows out. Connect the two conveniently and cleanly. Probably a hose from the auto parts store, and 2 hose clamps. I can take a picture of my setup in the morning.

Pretty easy. I'm not sure if there is anything up in front on an 88. Just use common sense. If I remember correctly, I had to purchase a formed hose and cut it down to the correct length.

 

[roger longman]

Hi vetteboy, much appreciated. Thank you. Roger.

 

[vetteboy86]

Have you located the two coolant lines connected to the throttle body?

 

[roger longman]

Hi vetteboy, I can see the hose to the heater but not the other one ! Roger. DUH !! I see it. OK. there's another thread on this so i guess we should go to it. What do you think ?

 

[vetteboy86]

Roger,

If you see both coolant passages it is easy to see what you need to do now! View my response in that thread, and keep us posted on what you decide to do!!

 

[roger longman]

Roger,

If you see both coolant passages it is easy to see what you need to do now! View my response in that thread, and keep us posted on what you decide to do!!

Hi vetteboy and all, Using what I have left of a brain I looked up some old threads on the EGR and TB cooling. The latter is quite contraversial,it seemed to me,so I'm waiting for some more input. Threads on the EGR concluded that blocking it off caused the car to run badly. How come mine is running better ? Idle is a bit rough but I'm getting over 17 mpg in town up from 13 and I can run in OD. Please HELP. Roger.

 

[Schrade]

Did you ever do the EGR test? What did it do?

I will do as you suggest when I get the chance.

Time to start over again. With COMPLETE diagnostic BASIC specs.

Can you post the name of the 'vette specialist', and business #? TPI DOES like low rpm's, so this 'specialist' ISN'T... Never mind any of his 'words'. Words go away when the sound stops. Get him to show you stuff.

Someone finally asked about codes (8 posts - ;squint: ). I'm bettin' you have one.

If not, do the basics - air, fuel, fire.
Air filter clear?
Fuel - static and dynamic pressure ok? (you NEED TO know - search my posts for FP tests).
Fire. Plugs and wires okay? Properly routed? Open the hood, running, PITCH dark, and look for the 4rth of July around the wires. Get a water spray bottle, and spray around the wires, and look for more discharge uh huh...

One problem is never a problem. But it will sooner or later DIRECTLY cause another problem. And when that happens, doing diagnostics is NEAR IMPOSSIBLE.

 

[roger longman]

Hi Schrade, I reconnected the EGR to get smogged. The NO reading showed the EGR to be working, so I think something is telling it to open at low RPM and causing the miss, also the HC reading was on max. 112 at 15 MPH. Any ideas ? I put up another thread " not EGR, why the miss " yesterday but no one has replied. Thanks Roger.

 

[Schrade]

The NO reading showed the EGR to be working, .

That is not right.

Your burn/tune is off substantially. Your fuel trim is way out of spec. Check your BLM fuel trim, and it is no where near 128, which is spec.

Maxed fuel means elevated HC count, and this ALWAYS drops oxide of nitrogen way down in the emission readings - search 'emissions' posts in this forum, where readings are posted, you'll see what I'm talking about. Low NOx count is NOT because the EGR is doing anything - promise.

I garauntee your fuel trim is too high. This makes a rich burn, AND A LOW BURN TEMP, WHICH IS WHAT IS MAKING NOx COUNT LOW. EGR also lowers burn temps by adding exhaust gas, but trust me, that's not what's going on with yours... I don't even think EGR functions (opens) at idle.

 

[roger longman]

That is not right.

Your burn/tune is off substantially. Your fuel trim is way out of spec. Check your BLM fuel trim, and it is no where near 128, which is spec.

Maxed fuel means elevated HC count, and this ALWAYS drops oxide of nitrogen way down in the emission readings - search 'emissions' posts in this forum, where readings are posted, you'll see what I'm talking about. Low NOx count is NOT because the EGR is doing anything - promise.

I garauntee your fuel trim is too high. This makes a rich burn, AND A LOW BURN TEMP, WHICH IS WHAT IS MAKING NOx COUNT LOW. EGR also lowers burn temps by adding exhaust gas, but trust me, that's not what's going on with yours... I don't even think EGR functions (opens) at idle.

Hi Shrade, I bow to your knowledge but the low NOx count was not at idle, 15 and 25 mph. Do your comments still apply. What do I do next ? Thanks Roger.

 

[Schrade]

Knowledge is useless unless the problem gets fixed. (see sig).

And I don't know where to go next, except to square 1, probably FULL fuel pressure tests. I DO believe you're gettin' too much fuel, as the emission scan showed.

Can the inspection shop give you a print out of the scan data? Maybe give them a call and check. Maybe go in and offer one of the guys a few bucks to do it. A speed shop will do it too, but might cost a few more bucks, then post it up here. I think that's the only way to confirm that your fuel trim is probably too high...

That could also be what was causing the stumble at low RPM 4rth gear, and I suggested in post #37:

It's possible that your fuel pressure regulator is NOT responding to vacuum.

When you drop hammer, the vacuum increases, and the vac line to the fuel pressure regulator makes the regulator HOLD extra fuel pressure, instead of returning it to the tank. This is acceleration enrichment. I was thinkin this might be your problem.

BUT, you say the problem manifests when it shifts to OD. When that happens, vacuum DECREASES, and fuel pressure is supposed to DECREASE. This is de-celeration enleanment (that's what Ray C Bohacz calls it in a TPI tech article). LT1 is probably the same.

SO, if your gettin' extra fuel still, when the revs drop for the 3-4 upshift, the motor will BOG DOWN.

It's a stretch, but it IS possible. There is a spec measurement in FSM for vacuum response in fuel pressure test. I do not know what the response is in LT1, but I can find the tech article for L98. Which do you have?

edit:
Next time you're FULLY warmed up, get to about 35 - 40mph, and take it to about 3/4 throttle - not all the way down. Revs won't drop enough at the shift to allow the stumble, IF the vacuum/regulator issue is what's going on. If it DOES stumble, I'm probably wrong.

Either complete fuel pressure tests, or a scan / log printout...

 

[roger longman]

Hi Schrade, Mine is the L98. I'll get on to the smog shop tomorrow and find out if they'll give me a copy of the scan. Thanks. Roger.

 

[roger longman]

Knowledge is useless unless the problem gets fixed. (see sig).

And I don't know where to go next, except to square 1, probably FULL fuel pressure tests. I DO believe you're gettin' too much fuel, as the emission scan showed.

Can the inspection shop give you a print out of the scan data? Maybe give them a call and check. Maybe go in and offer one of the guys a few bucks to do it. A speed shop will do it too, but might cost a few more bucks, then post it up here. I think that's the only way to confirm that your fuel trim is probably too high...

That could also be what was causing the stumble at low RPM 4rth gear, and I suggested in post #37:



Either complete fuel pressure tests, or a scan / log printout...

Hi Schrade, The only scan they knew was the one they gave me. I'll first give the results from year 2000 for comparison to now......HC. max 136. AVE 30. MEAS 52. CO. MAX 0.72. AVE 0.14. MEAS 0.01. NO. MAX 1473. AVE 206. MEAS 682..... now HC. MAX 112. AVE 31. MEAS 112. CO. MAX 0.72. AVE 0.10. MEAS 0.02. NO. MAX 778. AVE 237. MEAS 98..... Does this bear out your suspicions ? Many thanks. Roger.

 

[Schrade]

Hi Schrade, The only scan they knew was the one they gave me. I'll first give the results from year 2000 for comparison to now......HC. max 136. AVE 30. MEAS 52. CO. MAX 0.72. AVE 0.14. MEAS 0.01. NO. MAX 1473. AVE 206. MEAS 682..... now HC. MAX 112. AVE 31. MEAS 112. CO. MAX 0.72. AVE 0.10. MEAS 0.02. NO. MAX 778. AVE 237. MEAS 98..... Does this bear out your suspicions ? Many thanks. Roger.

I think that's a gas chromatography scan of exhaust gases. They tell that the burn is rich, but we're not knowing why. We need a PCM scan of the ALDL interface, that will give info like this:

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 %

Notice #17 and #18. They are reference values - for an ideal fuel trim. BUT, this scan shows a bad burn which is not even the same degree of 'OFF' for both sides of the motor. Look at #15 and #16 - they aren't too close to 128, which they should be.

Then this guy changed a part that WAS bad, did another scan, and one value went to close to where it should be, from way off - see it? But he had other problems too. Notice the fuel trim didn't start to correct, because it was out of tune for so long, but IAC count DID start to correct #23/#24 next paragraph...

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 %

#23 and #24 are supposed to be close also. They are not. If they were the only 'OFF' readings, we could assume it was a vacuum leak. While working on that thread, another guy did find a vacuum leak in his vette - the ONLY one who I know ever found a leak with my vac leak technique. That one was in the intake air control pintle gasket.

Notice also the discrepancy of injector pulsewidth between left and right side #19 and #20? That is most likely spark plug, or wire, or, GULP, opti...

We really need to see a scan to know why the exhaust gases are as they are. Otherwise, we can only guess. And with no codes, that's not easy.

 

[navy2kcoupe]

What about the last plug that was replaced by itself? Wasn't that one "carbon fouled"? Isn't running rich a cause of carbon fouling? If that is the only plug that was fouled, then I would suspect THAT injector of being the one making the rich burn condition. Just a thought. But that car needs to be scanned by someone that knows what they're doing, or all we're going to be able to do is guess..............
Andy
This is an interesting read..............

 

[roger longman]

Hi Schrade, Navy, I would think that if the no.6 cylinder plug,which was fouled is the fault of an injector, plugging off the EGR would not have cured the miss. Am I wrong on this ? Many thanks. Roger.