1994 5.7 TBI Runs only at Hi-RPMs

[Blazing95]

While driving my 1994 Blazer Silverado 5.7 TBI 89,000 miles
(No Check engine light).
As soon as I started to brake-de-accelerate the truck stumbled and died, started back up and continued to drive, the only way to drive was to double foot the brake while giving it gas to get it going. Since the tank has been run low on gas numerous times I felt it was the fuel pump so replaced the complete sending unit and a new Carter fuel pump. It starts and runs fine and continues to stay running only if I give it continuous gas as Hi Rpms after I take my foot off the gas it will run for a minute then stumble and die and then fire right back up. Cold or Warm it will run on its own then stumble and die-What am I over looking-?

[j cAT]

not much to go on so start with vacuum leaks,check vacuum lines use a vacuum guage..check your fuel pressure make sure it is correct...most likely it is ok because it does run ok off idle..

it is possible the EGR valve may be defective ..if it is very old / original replace...this valve if it leaks at idle will cause poor running at the low RPMS...

[Blazing95]

jcat Thanks for the quick response. I agree not much to go on too. I have looked at all the hoses they are either plastic or rubber all look good No Cracks dried out etc..

I will look at the egr and post back. When cold the truck runs at hi rpms then down kicks when warmed up and then just stumbles and dies.

I'm working on the pressure for the fuel but feel it is not that as it starts and runs.
again No Check engine light and it does have an obd II not an obd I as it was manufactured in late 1994. Any other suggestions-?

[j cAT]

Originally Posted by Blazing95

jcat Thanks for the quick response. I agree not much to go on too. I have looked at all the hoses they are either plastic or rubber all look good No Cracks dried out etc..

I will look at the egr and post back. When cold the truck runs at hi rpms then down kicks when warmed up and then just stumbles and dies.

I'm working on the pressure for the fuel but feel it is not that as it starts and runs.
again No Check engine light and it does have an obd II not an obd I as it was manufactured in late 1994. Any other suggestions-?

if it starts good , idles good at cold start then when it starts to heat up the engine has idle issues this looks more like the EGR...the egr will not operate on cold start ..after warm up then it is allowed to open..so check this out that the valve is clean and does not stick ..remove and inspect..Like I said if it is original just get a new one...after all these years the power head spring gets weak..

check for vacuum leaks with a guage...this is much better than looking at the lines and also you will have a vacuum reading which tells a lot about how your engine is operating...

obd I-II with the 1994-95 years this was a transition period so It is not clear cut which vehicles are I/II..

[Blazing95]

hey jcat-Pull cleaned and finally broke down and replaced the EGR no change still stalls runs good at cold or 1200 rpms then downkicks,stumbles and dies this time it does not restart until after an hour or so. Looking at how the cone spray is spraying in the bowl- took it upon myself to replace the TBI injectors figuring it was a fuel problem- Wrong again- Starts good runs at 1200 rpms foot off the gas pedal idles shortly stumbles and dies- does not retart- Begining to start to look at the sparking situation Coil, ICM, Cap Rotor- Your thoughts which one to start with first-?
By the time I/m done and broke might need to refinace again-lol
The vaccuum is 14"

[jdmccright]

Check the engine temp sensor in the water neck. If it is faulty it will continue to run in open loop mode even after the engine is warmed up. Also a common cause of no-start when warm.

[Blazing95]

jdmccright Thanks for your input all information is helpful as this is why we are on AF. I did a manifold gasket on the truck last year and broke the coolant sensor so as of July 09 it is a new one. I double checked this when this mysterious stalling started. Yes Sir I will check it again as I'm saving my last gallon of $2.79 gallon of gas for torching her and collecting the insurance-Just kidding, I have owned it since new and had never had a problem i couldnt solve until now..

[jdmccright]

A BIC lighter's only 50 cents...and a more believable cause as an "accidental" accelerant...I'm just sayin', save what you can...kidding.

Might check the IAC valve too...that can cause rough idle and poor running.

[j cAT]

Originally Posted by Blazing95

hey jcat-Pull cleaned and finally broke down and replaced the EGR no change still stalls runs good at cold or 1200 rpms then downkicks,stumbles and dies this time it does not restart until after an hour or so. Looking at how the cone spray is spraying in the bowl- took it upon myself to replace the TBI injectors figuring it was a fuel problem- Wrong again- Starts good runs at 1200 rpms foot off the gas pedal idles shortly stumbles and dies- does not retart- Begining to start to look at the sparking situation Coil, ICM, Cap Rotor- Your thoughts which one to start with first-?
By the time I/m done and broke might need to refinace again-lol
The vaccuum is 14"

the vacuum should be 19 inches ...with 14 inches you have a leak or the timing is retarded...recheck all the lines going into the intake / check intake gaskets ....usew propane gas to reveal the leak[s]....

[Blazing95]

jcat Will the propane explode after I light the Bic and take the truck to the unknown-?
Mystery solved... I have never seen this one before as it either fails completely but never partially. (Stay tuned a little history) After replacing the fuel pump and the sending unit and $165.00 the truck fired right up an ran, as it warmed it shut off I never checked the ignition thinking it was a fuel problem all along. As it ran I noticed 1 TBI injector dribbling so I replaced both TBI Injectors. The truck fired up and ran as it got warm shut down with no restart again never checking for spark.This time and 22 degrees outside decided to check for spark on no restart-No Spark-pulled the cap and saw the posts totally built up with corrosion, replaced cap and rotor at the same time while checking the resistance on the coil. Coil good New cap and rotor and assembling all to see if spark Not even a little spark-Frustrated and morally bankrupt you guessed it replaced the ignition module re assembled all and Holy Crap we now have a living breathing 5.7 back on the road.I have never seen an ignition module go partially bad effecting what seems to be a fuel issue. Fellow gear heads, stick to the basics while diagnosing your current issues and ask others till you feel that you have exhausted the possible corrective action. I find AF a very useful tool and the input along with pictures showing the fixes has been priceless. Thanks all for your support and your humor while getting to know my truck more.
PS the wife is much happier now that I'm not using her truck

[j cAT]

Originally Posted by Blazing95

jcat Will the propane explode after I light the Bic and take the truck to the unknown-?
Mystery solved... I have never seen this one before as it either fails completely but never partially. (Stay tuned a little history) After replacing the fuel pump and the sending unit and $165.00 the truck fired right up an ran, as it warmed it shut off I never checked the ignition thinking it was a fuel problem all along. As it ran I noticed 1 TBI injector dribbling so I replaced both TBI Injectors. The truck fired up and ran as it got warm shut down with no restart again never checking for spark.This time and 22 degrees outside decided to check for spark on no restart-No Spark-pulled the cap and saw the posts totally built up with corrosion, replaced cap and rotor at the same time while checking the resistance on the coil. Coil good New cap and rotor and assembling all to see if spark Not even a little spark-Frustrated and morally bankrupt you guessed it replaced the ignition module re assembled all and Holy Crap we now have a living breathing 5.7 back on the road.I have never seen an ignition module go partially bad effecting what seems to be a fuel issue. Fellow gear heads, stick to the basics while diagnosing your current issues and ask others till you feel that you have exhausted the possible corrective action. I find AF a very useful tool and the input along with pictures showing the fixes has been priceless. Thanks all for your support and your humor while getting to know my truck more.
PS the wife is much happier now that I'm not using her truck

thats great you have this under control...
with your distributor issues ....these distributor modules fail at 175,ooomi area usually ...BUT if there is arcing in the coil / wires , cap this module fails as voltage surges kill it...

with the propane gas ....you get a small hand held propane torch. then add on the end where you would light it a hose about 3-4ft long ...

then you open the torch valve slightly so you can smell the propane not too much,,,this is because it will mess up finding the exact location of the leak.....

do not do this inside a closed interior space ....outside in the cold winter you will not explode...

what will happen is the RPM will rise when the leak is sucking in the propane....so having a tach when doing this you can check out your engine for vacuum leaks with out using carb cleaner which is bad for you gaskets/rubber and your lungs....

if you direct this propane into the air intake you will see a rise in the rpm...there will be a slight delay in this response..

14inches is too low however your timing may have caused this low reading...