Why the miss ?

[vetteboy86]

It is hard for me to remember what it was like before headers. With the TPIS longtubes I can get every plug out and back in, and it takes about 10 minutes compared to an hour it used to.


I ran into an issue about 3 months ago. I have been extensively tuning my car, and I coincidentally I started a new series of files, changed the source code to try something different. It was than I started getting mis fires, no power. I beat myself up over it for days. Put the old code back in, same thing. I immediately starting attacking the most difficult problem to solve instead of looking at the basics. So my bro came over to give me a hand, and found that one of the plug wires had come out of the holder and was laying against the header. It was fried. Repaired the plug wire, and she was like new!!

 

[Vettefan87]

I agree about the headers that makes a night and day difference.

 

[So Bay Vetter]

he came up with a "Snap ON" plug socket with a swivel base I had not seen nor heard about before.

Any chance of a part # or size on that Snap On swivel socket?

 

[roger longman]

Hi Sobay, I'll ask him next week. Roger.

 

[roger longman]

Hi Sobay, part number is FSX 320 for 3" and FSX 620 for 5". I used the 3". Roger.

 

[roger longman]

OK, car is running much better BUT it's still not completely happy at low revs in OD. What have I missed ? Roger.

 

[vetteboy86]

What did you do that made it run better?

 

[roger longman]

Hi vetteboy, I changed the no. 4 plug, that I had considered not to be the problem as the rest had been in such good condition. I borrowed a Snap On flex socket and when out, found it to be completely carbon fouled. Roger.

 

[LT4man]

Are you sure the #4 ignition wire is good? Sounds like you need to check that one carefully.

No reason for a spark plug to foul, unless the ignition wire is no longer sending the current to the plug. Could also be the contact inside the distributor cap.

AFTER REPLACING THE #4 SPARK PLUG, DON'T FORGET TO SAVE THE !

 

[vetteboy86]

Hi vetteboy, I changed the no. 4 plug, that I had considered not to be the problem as the rest had been in such good condition. I borrowed a Snap On flex socket and when out, found it to be completely carbon fouled. Roger.

Roger,

After checking the spark plugs, you found one that was fouled while the others were not?

 

[Schrade]

OK, car is running much better BUT it's still not completely happy at low revs in OD. What have I missed ? Roger.

I don't believe it has anything to do with OD.

Have you done fuel pressure test? If not, you need to do it - static and dynamic tests.

Static = key on, not started. Note (and write down) initial pressure immediately when rail gets primed. Note pressure bleed-down times, and write down drop in10 or 15 second increments, or 30 if slow dropping. This could tell leaking injectors due to weak pintle springs. You said 18 ohms all 8 injectors. Might be right, but I've never heard of anything over 16...

Then do dynamic pressure test. Idle pressure with vacuum hose OFF of pressure regulator, then with vacuum hose ON fuel pressure regulator.

THEN, MOST IMPORTANT FOR YOUR CASE I THINK, with vac hose still on FPR, pull the linkage slowly, and see if pressure INCREASES with RPM's. This is acceleration enrichment, and I think you ain't gettin enrichment.

Even tho' fuel is gettin' pulled with pedal down, vacuum makes FPR close up a little, and allow pressure to build up by cuttin' off return flow.

I think you need to do nothing else at this point.

Tranny problems would probably let the revs go UP, due to slippage, and increasing revs would HIDE a misfire.

 

[roger longman]

Roger,

After checking the spark plugs, you found one that was fouled while the others were not?

HI vetteboy, Yes I'd previously changed the other 7 plugs and all 8 wires. Thanks. Roger.

 

[roger longman]

Hi Schrade. It isn't a tranny problem, it was changed by Dick Guldstrand, about 6000 miles ago. I take your point on higher rpm hiding a miss. It's not a fault with the OD it's a low revs/ high gear problem. OOPS, didn't realise I should zero the meter before ohming. just rechecked they're all at 16. Your next 3 sentences, begining Then and ending flow have got me, at the moment. But you seem pretty sure this is it, if so what do I need to do ? Many thanks for your help. Roger.

 

[So Bay Vetter]

Hi Sobay, part number is FSX 320 for 3" and FSX 620 for 5". I used the 3". Roger.

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

 

[roger longman]

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

Sure, Tues through Fri, for coffee or WHY between 2pm and 5 pm, would be good for me. Are you thinking at main and grand, what's good for you ? Roger.

 

[vetteboy86]

OK, car is running much better BUT it's still not completely happy at low revs in OD. What have I missed ? Roger.

I did a test tonight. I was able to get my car run at 38 mph and 900 Rpm's. Absolutely no issues whatsoever.

What does that mean??? Well I guess it means that I do not have the same issue you have.

Can you physically see it on the tach? What you are describing that is!

 

[Schrade]

Your next 3 sentences, begining Then and ending flow have got me, at the moment. But you seem pretty sure this is it, if so what do I need to do ? Many thanks for your help. Roger.

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.

 

[roger longman]

Hi vetteboy, still can't do as you can, it "stumbles" but not as badly as before the last plug was changed. No it doesn't show on the tach. Thanks Roger.

 

[roger longman]

Hi Schrade, Yes mine is the L98. Are you saying that I should be in OD, take it to a speed just below that at which it would auto shift up into OD and then give it 3/4 throttle and see whether revs dip ? If so I'll try it tomorrow and report back. Thank you. Roger.

 

[roger longman]

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.