Help C4C5Special, Grumpy, everybody

[SPANISHVETTS]

Thunder is making me nuts…Help!
Runs great when cold. When hot stalls when you accelerate from idle or low speed, backfires in exhaust and intake. After sputtering for a bit it will run at 2000RPMs or more but not good.
Changed: fuel and air filters
Checked fuel pressure: 40 – 44 lbs.
Checked injectors resistance: all within tolerance.(Could they fail when hot and test ok by the time I can disconnect and test them?)
Swapped distributor with a known good unit. New plugs and plug wires.(old plugs were clean and brown, looked a bit lean but not bad)
Chassed all of the ground wires – can’t find a bad one.
Compression appears OK. I only have a static gauge so can’t test bleed down. Engine has 18 K on serious 383 build – uses no oil.
Checked for gasket leaks and checked the coolant for exhaust gas. Everything looks OK.
Bought an AutoXray 6000 scanner because the team does not have an OBDI module for the SnapOn.
Here is what it caught during 10 seconds while Thunder was spitting and backfiring.
Time.... 0 ....+1sec.. +2sec.. +3sec.. +4sec.. +5sec . +6sec . +7sec . +8sec . +9sec
RPM. .1028... 813..... 894..... 1148..... 1238..... 882..... 811. . . 1233. . . 864... 1028
TPS.. 1.17V. 1.17V ..1.17V.. 1.25V.... 1.37V.. 1.37V .. 1.35V .. 1.70V . 1.70V . 1.72V
Speed 0MPH. 0MPH.. 0MPH... 0MPH... 2MPH... 2MPH. . 3MPH . 2MPH . . 2MPH. 2MPH
O2... . 28mV .20mV.. 24mV.. 32mV. . 24mV.. 20mV . 24mV. . 32mV . . 24mV 28mV
MAP. . 3.52V .4.05V.. 3.70V.. 3.66V. . 3.99V.. 4.25V . 4.33V .. 4.48V . . 4.66V 4.60V
IPW* 4.5mS 6.1mS.. 4.6mS. 4.9mS.. 5.2ms. 5.9mS . 5.6mS . 6.0nS . . 6.2mS 6.5mS
RLF*.. Lean. .Lean... Lean. ...Lean. . .Lean.... Lean. . Lean . . Lean. . . Lean.. Lean
Loop Closed. Closed .Closed. Closed. Closed. Closed. Closed . Closed . Closed Closed


*IPW injector pulse width
*RLF rich lean flag
After pulling my hair out I am down to fuel pump or injectors. The injectors are bosh 28# with 18K miles and the fuel pressure looks OK. The injectors rattle like they are firing so I have ruled out the quad drivers.
WHAT HAVE I MISSED?

 

[G Winter]

Something with EGR ? oxygen sensor ? I see lean. Sock inside the tank,the fuel pressure should show it. Hope someone knows how to use the IPW readings, that may show you to your answer.

Glenn


an after thought , you couldn't have an intake valve hanging open ?

 

[bradfordsvettes]

the O2 sensor voltage is very low almost seems stuck low indicating a lean mix , and the map is higher than I think it should be even with a modified motor which is indicating low engine vacum .is it possible that you have a bad valve spring this could be checked with a vacum gage , the O2 may need attention you could add some propane to the mix and see if the O2 starts to react .looks llke it is on
with the chase hope this adds some insight Steve

 

[vetteboy86]

Extreme Vacuum leak maybe??? I had a similiar problem with my motor with the O2 sensor never changing states. I eventually put a heated O2 in and that fixed the problem. Could it be time for a new O2 sensor?

 

[SPANISHVETTS]

Thanks for the input. It looks like fuel starvation but why? The fuel pressure gauge I used was a cheap one. Is it possible that the pressure drops so fast that the gauge does not show the drop?
The O2 sensor is heated. When the engine is running ok the output is correct and swings from lean to rich as it should.
I will monitor the coolant temp today and see at what temp it starts acting up. On a cold day it runs perfect. That means 3 days in February here in the south of Spain.
At cruising speeds it runs OK.
AutoXray owes me the com cable for the scanner to laptop link so I can not capture more than 10 seconds at a time until they send me the cable.ad

the O2 sensor voltage is very low almost seems stuck low indicating a lean mix , and the map is higher than I think it should be even with a modified motor which is indicating low engine vacum .is it possible that you have a bad valve spring this could be checked with a vacum gage , the O2 may need attention you could add some propane to the mix and see if the O2 starts to react .looks llke it is on
with the chase hope this adds some insight Steve

Steve, in all my years of wrenching I have never had a bad valve spring that did not cause more serious problems but 80% of my mechanicing has been on bikes and they usually burn or drop a valve so fast that you never knew the spring was going south before the thing went bang. Would a bad spring only act up when the motor is hot and run better at higher RPMs?
An open intake valve would explain the backfire in the intake manifold but with those wide injector pulse widths where is all of the gas going? Shouldn’t the good cylinders be firing and the O2 going rich from the extra gas?:confused

If this helps any it backfires through the exhaust as well as the intake. The cats are new. I removed them and tried it with open exhaust. This made no difference to the problem but set of the alarms on every garage on the street.

Thanks everybody and please keep up the input. It is a real B!tc# working on a Vette here in Euro-land where every part is 3 days away…. Lucky that our cars do not fail often. In 5 years this is the first time that Thunder has given me any serious trouble.

 

[gmjunkie]

Something with EGR ?

Glenn

I believe that Glenn is on to something!!!!! I think you have a piece of carbon bouncing around in EGR or it is just stuck!! I've had problems with carbon getting stuck in EGR on these and other GM cars!! 4.3's are extremely bad about it!! Remove EGR and Clean it!! I'd almost bet there is a piece of carbon stuck in it!! That will lean it out!! And make um run like you explain!!I usually start them up with EGR off to blow carbon out of passages in intake!! Man the Ignition switch because it will take off Like the Big Bird with out giving it any throttle!! I then use 2 gaskets with a stainless Faucet screen sandwiched between them in the inlet port to break up any carbon that remains in intake so it will pass through EGR without getting stuck in the pintail!!

 

[SPANISHVETTS]

Thanks junk but I have no EGR. Bigmouth base plate with the EGR holes blocked off and the headers do not even have the EGR pipe. Could it be leaking around the blocking plate? Wouldn’t that cause a high idle?

Could the injectors be ok cold and crap out when hot? If so why would they fail at 1000RPMs but work at 3000 RPMs?

I am leaning towards a valve related problem. It is just hard to believe that something has crapped in 18K miles. Everything in the 383 came from GMPP.

What really throws me off of the valve theory is it running great when it is cold. This throws out a worn cam lob but would a weak valve spring act up more when hot than cold?


I am :W and going:crazy .

 

[gmjunkie]

Thanks junk but I have no EGR. Bigmouth base plate with the EGR holes blocked off and the headers do not even have the EGR pipe. Could it be leaking around the blocking plate? Wouldn’t that cause a high idle?

Could the injectors be ok cold and crap out when hot? If so why would they fail at 1000RPMs but work at 3000 RPMs?

I am leaning towards a valve related problem. It is just hard to believe that something has crapped in 18K miles. Everything in the 383 came from GMPP.

What really throws me off of the valve theory is it running great when it is cold. This throws out a worn cam lob but would a weak valve spring act up more when hot than cold?


I am :W and going:crazy .

We had a similar problem on my daughters Z-71 after we put it back together!! It would run cold and around town 100%!! After it ran 30 min on Highway it would miss, backfire, and pop through throttle body at low speed!! I had the valves too tight!! Backed them off 1/4 turn and it runs Like a Champ now!!(But then a Champ was a Studebaker and weren't worth a Flip!!)

 

[SPANISHVETTS]

(But then a Champ was a Studebaker and weren't worth a Flip!!)

I got my first kiss from Linda Sanders in the back seat of her brother's Champ...I still consider that Champ America's first Sporting car.

Thunder ran great for 18K on the rebuild. The problem acted up once last fall and went away so I wrote it off to bad gas. All winter it ran great and then last month when it got hot the problem came back 10 times worse.

I am up to my eyeballs today... the valve covers will come off Thrusday.

 

[c4c5specialist]

HI there,
I too would be leaning towards a vacuum leak.
Somethings leaking, causing your MAP issue with lean o2 sensor voltage.
Allthebest, c4c5

 

[8388]

My vote is with a lean condition due to a vac leak. Spray some brake cleaner around the usual suspected areas. If the idle jumps up, you have pinpointed your vac leak.

 

[toptechx6]

Another thought

If the car runs well when cold it seems to me it is getting enough fuel since a cold engine requires a richer mixture. Keep in mind the O2 sensor responds to oxygen NOT fuel, it has no idea how much fuel is present just how much O2. Once you have a misfire, the oxygen (remember air is most of the mixture) enters the exhaust with the unburned fuel, the sensor sees this as a "lean" condition and adds fuel (wide pulse width). I'm not familiar with your engine, is spark advance controlled by the distributor or the ECM?

 

[SPANISHVETTS]

Up date

Pulled the valve covers off and checked the rockers. (Why are the AC compressor and Alt. bracket in the way by less than 1/4¨? Having to moving the compressor to pull a valve cover is always silly but for 1/8¨ of clearance is just plain dumb.ad) All of the rockers were about 1 turn loose except #6 which was 1 full turn tight.:confused

It seems to be running better but the weather cooled off. I will not know for sure until it goes over 90 again.

Thanks to the scanner I found another problem. Thunder is running much too hot. I run Evans waterless coolant and it has always run at 190º. The scanner showed it going over 220º. The fans are coming on at 190 which is the way the chip is programmed.

I pulled out the thermostat and put it in the kettle. It is stuffed; opening and closing allover the place. On one cycle it opens OK the next time it does not open until the water is boiling. I have had a few bad thermos but never one with a mind of its own.:W

The beast runs fine until the coolant temp goes over 215º. It could be that the problem was always there and just never showed up until the thermostat crapped out. That could also explain it acting up one time last year and running great for 6 months.

A passing thought. Could it be that a gasket or some of ¨the right stuff¨ on the manifold are breaking down at high temp but working when it cools off? If that is the problem then it is sure to get worse and leave me calling AAA.

 

[gmjunkie]

Keep a Eye on # 6!! You may have a valve seat or seats going South!~!

 

[plumcrazy]

All of the rockers were about 1 turn loose except #6 which was 1 full turn tight.:confused


i would look for why your rockers where all out of wack.


i run zink additive in my 383 with valvoline #211 20-50 oil. my builder is very respectable and told me to run the additive with the my heavy springs or you will tear the lobes off the cam.

 

[plumcrazy]

oooooooooooooooooooops this was an old post.