Bucking only when Power key is OFF

[MuRCieLaGo]

Hi guys,

As I mentioned on another thread, I just did a 200 miles trip today, always with Power key on (should I?). Tonight, I tried the car with the Power key in normal position and after some seconds of WOT, the car is bucking like if it was running on 6 cylinders. Problem is there only with Power key OFF, and goes away after a few seconds of the Power key going in the FULL position.

Any idea? Car pulls perfectly strong with the Power key at FULL position (except that it seems to be pretty slow at RPMs under 4,000, maybe it's normal though).

(In the video, it starts to buck in 4th gear, until the end).

https://www.youtube.com/watch?v=umhXEyiK9W0

[tomcat]

hi, that happen to me. Changed coil packs. runs fine now.


Manfred E....
91 #906

[MuRCieLaGo]

Thanks Manfred, makes a bit of sense. And I see some cheap coil packs on eBay:

http://www.ebay.ca/itm/Cadillac-Chev...-/271705596942

Do you recommend it or MSD is worth it?

[secondchance]

Originally Posted by MuRCieLaGo

Thanks Manfred, makes a bit of sense. And I see some cheap coil packs on eBay:

http://www.ebay.ca/itm/Cadillac-Chev...-/271705596942

Do you recommend it or MSD is worth it?

I bought a cheap set off ebay and they turned out to be Chinese made AC Delcos.

I am not so sure if I want to use them.

[Paul Workman]

"Bucking" in NORMAL mode only, but NOT in FULL ENGINE POWER, (key ON) suggests;

1. one or more primary injectors = "kaput"
   
2. primary fuel pump at issue.

Whereas, missing (bucking) at WOT w/ power key ON, suggests fuel issue and/or ignition: which is NOT what you've described.

So, IMO, NOT missing at WOT, but miss-firing in NORMAL rules out ignition (plugs/coils) for now. (FYI, the most severe load on ignition system is at WOT. If coils or plugs or wires were going to fail, that would be when it was noticed - the opposite of what you've detailed fur us here. Just sayin.)

With the secondaries ON and at WOT, the secondary fuel pump is running, AND the secondary injectors are chiming in too, which could be masking a bad primary pump and possibly one or more marginal injectors too (or check-valve issue like I had once).

If the fuel pressure checks out (~45psi in NORMAL, low throttle mode and 52 or closer to 55 psi at WOT) you will want to measure the resistance across each of the injectors while they are HOT (Ignition key OFF).

Resistance should be uniform across the lot, ± an ohm from average. [averages depend on manufacture. The may average, 12, 14. or 16 ohms ± 1.] Typically, when they cause issues, the resistance of a bad injector is 3 or 4 ohms lower than the average. For example, if the group reads approx 14 ohms, but one reads (for example) 11 or less, it is marginal and probably headed "south". But, by the time it is reading closer to 9 (in this example) or lower, it is NOT firing properly; replacing the entire lot is the recommended solution.

For specific instructions on "how to" make these measurements, you can refer to Dynomite's sticky (top of the TECHNICAL section and click on the diagnostics for verifying fuel pump operation, as well as how to measure the injector resistance.

Between pre-93 injectors and old fuel pumps, this is by far the lion's share of the problems you've described If this all checks out, then it is something more involved and we'll have to go from there. (Note: I'm not saying coils are NOT also and issue, but at this point, they are not high on the list, due to the symptoms you describe. We'll see tho, I recon...

Let us know how it goes~

[MuRCieLaGo]

Originally Posted by Paul Workman

"Bucking" in NORMAL mode only, but NOT in FULL ENGINE POWER, (key ON) suggests;

1. one or more primary injectors = "kaput"
   
2. primary fuel pump at issue.

Whereas, missing (bucking) at WOT w/ power key ON, suggests fuel issue and/or ignition: which is NOT what you've described.

So, IMO, NOT missing at WOT, but miss-firing in NORMAL rules out ignition (plugs/coils) for now. (FYI, the most severe load on ignition system is at WOT. If coils or plugs or wires were going to fail, that would be when it was noticed - the opposite of what you've detailed fur us here. Just sayin.)

With the secondaries ON and at WOT, the secondary fuel pump is running, AND the secondary injectors are chiming in too, which could be masking a bad primary pump and possibly one or more marginal injectors too (or check-valve issue like I had once).

If the fuel pressure checks out (~45psi in NORMAL, low throttle mode and 52 or closer to 55 psi at WOT) you will want to measure the resistance across each of the injectors while they are HOT (Ignition key OFF).

Resistance should be uniform across the lot, ± an ohm from average. [averages depend on manufacture. The may average, 12, 14. or 16 ohms ± 1.] Typically, when they cause issues, the resistance of a bad injector is 3 or 4 ohms lower than the average. For example, if the group reads approx 14 ohms, but one reads (for example) 11 or less, it is marginal and probably headed "south". But, by the time it is reading closer to 9 (in this example) or lower, it is NOT firing properly; replacing the entire lot is the recommended solution.

For specific instructions on "how to" make these measurements, you can refer to Dynomite's sticky (top of the TECHNICAL section and click on the diagnostics for verifying fuel pump operation, as well as how to measure the injector resistance.

Between pre-93 injectors and old fuel pumps, this is by far the lion's share of the problems you've described If this all checks out, then it is something more involved and we'll have to go from there. (Note: I'm not saying coils are NOT also and issue, but at this point, they are not high on the list, due to the symptoms you describe. We'll see tho, I recon...

Let us know how it goes~

It cannot be clearer than that. At the moment I gotta deal with that safety inspection (DRL and let's hope there is not a "etc..."). As soon as it is passed I'll work on that.

Thanks a lot Paul, I'll keep you updated on that issue.

[Paul Workman]

Originally Posted by MuRCieLaGo

It cannot be clearer than that. At the moment I gotta deal with that safety inspection (DRL and let's hope there is not a "etc..."). As soon as it is passed I'll work on that.

Thanks a lot Paul, I'll keep you updated on that issue.

Don't wait too long... I experienced a burnt intake valve which was "coincidentally" in the same cylinder as a very low resistance injector... It seems plausible there was a relation between the two, ya think?

Just sayin...

[MuRCieLaGo]

Paul,

Don't scare me now, please!

I'll do all that as a winter project. I'm enjoying my last days of driving here!

[MuRCieLaGo]

And today, SES light came on for maybe 3 seconds. Code 61.

[MuRCieLaGo]

And when I put the key ON, I don't hear the vacuum pump.