Why the miss ?

[Schrade]

Hi, At about 38mph in OD, accelerated 1st time, hesitated then kicked down (maybe I gave it more than 3/4 throttle). 2nd time, coughs then accelerates. 3rd time just accelerates. I know this isn't telling much except that it appears to be intermittent. If I accelerate through from a stop on between 1/4 and 1/3 throttle to cruising speed above 60 mph it's fine. But it's bugging me because it isn't RIGHT, as it used to be. Roger.

OD at 38 is not right. This is not electronic-controlled tranny - I don't know what could make a non-electric tranny shift early, except a gearing mod, or a tire size change.

Find a speed shop and get a data scan done. Get a printout and post up. I'm thinkin' in addition to a fuel pressure regulator, there's something else going on with the tranny here...

 

[vetteboy86]

Hi, At about 38mph in OD, accelerated 1st time, hesitated then kicked down (maybe I gave it more than 3/4 throttle). 2nd time, coughs then accelerates. 3rd time just accelerates. I know this isn't telling much except that it appears to be intermittent. If I accelerate through from a stop on between 1/4 and 1/3 throttle to cruising speed above 60 mph it's fine. But it's bugging me because it isn't RIGHT, as it used to be. Roger.

At what speed does the car go into overdrive? TCC issue?

 

[roger longman]

Hi, I did change tyre size to 245/50R 16 but I had this problem before that. The transmission was replaced but that was done by Dick Guldstrand, whom I would argue knows as much about the development of the vette as anyone. (see "The CORVETTE Fifty Year History by Randy Leffingwell". The references to Dick are numerous). MAYBE it's my inaccuracy, perhaps it's 40 mph not 38 but it's still not RIGHT. Thanks so much for your help on this. Roger.

 

[Schrade]

Try to get up with user PeteK in the other vette forums. I think he's probably the only one who could diagnose a tranny sight unseen.

If you can get a data log printout from a speed shop, and post it up for him, I'll put cash on his diagnosis.

I still think that a pressure bleed down to test the fuel pressure regulator might help rule OUT a few problems...

Did we ask if this twister idles perfectly smooth? No lopes?

 

[John Robinson]

And then

And then there is always the EGR valve intermittently sticking. And at least on my car it runs rough until I get out and use a screw driver to push the pintle while the engine is running. When you do this the engine will stop and the restart sometimes is a little rough but then it smooths out for a while.

 

[roger longman]

Hi, Will look at the pintle. Also remiss of not to say that the idle is lumpy as well. Sorry if my failure to mention it has caused any confusion. Thanks everyone for your time and patience. Roger.

 

[roger longman]

Thanx Roger! It appears we're neighbors. We should try to catch bkfast some time!

Hi SoBay, They're actually listed as FSXK 320A. Price is hefty, $56.00 + tax etc ! Do you still want to meet up ? Roger.

 

[Schrade]

Hi, Will look at the pintle. Also remiss of not to say that the idle is lumpy as well. Sorry if my failure to mention it has caused any confusion. Thanks everyone for your time and patience. Roger.

Lumpy? Like loping up and down?

 

[roger longman]

Hi Schrade, No, more like a one cylinder problem,but it has new wires and plugs. Maybe some injector cleaner might help ? Thanks. Roger.

 

[vetteboy86]

Not a lopey idle, just a rough idle? Does the motor shake? At what RPM does the motor idle at?

 

[Schrade]

Hi Schrade, No, more like a one cylinder problem,but it has new wires and plugs. Maybe some injector cleaner might help ? Thanks. Roger.

Probably not.

We're still at the pressure bleed-down test then. Procedure is posted in one of the previous posts. This is something EVERYONE needs to do as maintenance, even if there is NO problem. If there is a problem, do it as maintenance, and diagnostic.

Post up results...

 

[roger longman]

Not a lopey idle, just a rough idle? Does the motor shake? At what RPM does the motor idle at?

Hi vetteboy, Idles at 600 rpm, motor doesn't shake. Thanks. Roger.

 

[vetteboy86]

Refresh my memory, what RPM does this happen at? What speed as well.

 

[roger longman]

Hi vetteboy, 900 rpm, OD selected, 38-40 mph. Thanks. Roger.

 

[vetteboy86]

That seems early to me. Low RPM's as well. I may be spot off, but that is what I think.

 

[Schrade]

Hi vetteboy, 900 rpm, OD selected, 38-40 mph. Thanks. Roger.

Did you get a chance to check the fuel pressure tests?

 

[roger longman]

Did you get a chance to check the fuel pressure tests?

HiSchrade, Not yet ,I ,am going to do it this afternoon, hopefully. Thanks. Roger.

 

[roger longman]

Hi Schrade, Don't know if the mechanics gauge was off (it didn't have the core, so I had to take out the Schrader valve) key on 38 psi, running 39 psi. turned off went up to 46 psi, 10 secs -40, 20- 36, 30-33, 40-31, 50-29, 60-27, 70-26, 80-25, 90-24, 100-24, 110-23. Thanks. Roger.

 

[Schrade]

Hi Schrade, Don't know if the mechanics gauge was off (it didn't have the core, so I had to take out the Schrader valve) key on 38 psi, running 39 psi. turned off went up to 46 psi, 10 secs -40, 20- 36, 30-33, 40-31, 50-29, 60-27, 70-26, 80-25, 90-24, 100-24, 110-23. Thanks. Roger.

That's not too bad leakdown times, but not good either, depending on where it's leaking out, and there's a few different places it can go.

1) Back to the pump, if there's a check valve not seating well. Not too bad if the pressure maintains 40 when running (although THAT should be a few psi higher too). Does the pressure increase slightly when you rev it slightly? This is a reponse to vacuum increase.

2) Through leaky injectors. Didn't you post up you got new ones?

3) Fuel pressure regulator fault either in A) ruptured diaphragm letting gas into vacuum source (pull the vac hose off of fpr, and see if it's STRONG gas smell). If so, gas is pulling through that hose when running. AND, pressure will NOT increase when rev'ving (test 1 above); OR, B) weak diaphragm spring in fpr (which is not too bad, since running pressure is only slightly low). Pinch or cap return fuel line for this [static] test.

Any unmetered or poorly atomized fuel could cause slight stumbling at lower rpm under load, unless pedal is half or more throttle, and plenty of air is coming in quickly to compensate.

Double check bleeddown times with Jon Banner FIC injectors. He will probably agree. A scan/data log might also be necessary at a local speed shop, unless something else shows...

 

[roger longman]

Hi Schrade, thank you for your imput. No I didn't put in new injectors. Will check your list, to the best of my ability. Roger.