What's failing? Fuel Pump or O2 sensor?

[robertmee]

For the 2nd time in a month, my car dumped me again. This time I was prepared a little better and took more note of what was happening, plus I had installed a manual fuel pressure gauge on the rail.

Conditions: Both times have been after extended driving (an hours time). Weather has been mild, temps in the 70's. No WOT, just general cruising around.

Symptoms: Car stumbles and then dies. Turns over but doesn't fire. Let sit for a couple of minutes, cranks up and runs fine for a couple of miles, then repeats. After a few times, it will seem to recover and run fine, long enough for me to get it home. Next day, runs fine and strong.

Observations: This time I watched the SM O2s and CCs each time. On startup and takeoff, O2's oscillate as normal (50 to 750). When the car starts to stumble, the O2's begin to drop until they reach around 50. At this point, feathering the accelerator will keep it going for 10 to 20 seconds but then it dies again. Switching to Crosscounts, the crosscounts stop moving also when it is stumbling. While waiting to restart, key to on, fuel pump comes on as expected and fuel gauge indicates around 38psi. Then psi begins to drop until cranked.

It would seem because of the O2's reading 50 or less and the CCs stopping that the O2 sensor is going bad? Do these usually act up as a result of extended periods of heat? Because, it seems weird that letting it sit for a minute or two allows it to restart, but then after another minute or so, stumbles again. Can the O2 sensor also allow the car NOT to start immediately and require sitting like it does? That seems to point more towards fuel, but I dunno.

I also have a Walbro of the recall vintage so I'm always a bit wary of a failing pump. But it seems to prime fine and when not running for extended periods of time is strong on WOT and I get no KR. I also have kept the tank full (less an hour of driving) because I had heard that the pump can fail when not submerged and it heats up.

I'm appealing to the experts to help out here and explain what is going on and what I should look to fix.

 

[jasjamz]

Sounds like your ignition module. Does it have spark when it dies and you try to crank it?

 

[robertmee]

Well, both times it's happened at night, with the wife, so not a whole lot of time spent under the hood tearing things apart. Could very well be no spark on the attempted re-crank, but would loss of spark while running cause a low O2 number? I guess logically thinking about it, no spark and all fuel would certainly cause no O2.....hmmmmm

Still, I wonder if the O2 sensor is the culprit....I have noticed a very rich smell since owning the car and I tend to have BLMs that sometimes run in the 140's for no reason but most times around 130. I have no vac leaks, no intake leak and no exhaust leaks. I've read that richness and high BLMs could also be a bad or failing O2. Lastly, since the O2 comes into play in closed loop, how long on a heated engine does an Extender Chip reach closed loop? Is it immediately or after a minute or two. I wonder if that's why I'm able to run for a minute or so before it dies again because I'm running in open loop during that time??

 

[jasjamz]

I would stick to the ignition module. The O2 shouldn't make it stall, if it runs fine the rest of the time. I bet you have a loss of spark when it stalls.

 

[QUIP'6']

if our fuel pressure is rising 1:1 with the boost without falling of a wot then your fuel pump is ok. a o2 sensor is a cheap easy fix. i would start there!!! dont change to many things at once it could be costly and make things worse because new products can be bad even if new

 

[bmackinnon]

Sounds like your ignition module.

+1... almost certainly the ignition module... a failing o2 sensor won't leave you dead... symptoms are exactly that of failing module....

 

[robertmee]

I would stick to the ignition module. The O2 shouldn't make it stall, if it runs fine the rest of the time. I bet you have a loss of spark when it stalls.

Greeaaatt....My luck I end up with a $200 problem and not the $15 one :frown:

What's the defacto replacement? AC Delco like everything else? Best supplier?

Thanks for the diagnostic help!

 

[QUIP'6']

Greeaaatt....My luck I end up with a $200 problem and not the $15 one :frown:

What's the defacto replacement? AC Delco like everything else? Best supplier?

Thanks for the diagnostic help!

you can still get ac delcos on ebay for sale still in nos box

 

[DailyDrifter]

I guess logically thinking about it, no spark and all fuel would certainly cause no O2.....hmmmmm

It would cause low O2 readings because the air/fuel mixture didnt burn, therefore there is still Air and Fuel going across the O2 sensor which makes the O2 sensor read a lean condition.

I'm going to have to say it could be a ign problem. Your O2 sensor looked fine the other day, it was a little choppy, but still looked good. Let me know if you want my coil and Ign to try out.....................You know I wont need it anytime soon

 

[TurboTJT]

+1... almost certainly the ignition module... a failing o2 sensor won't leave you dead... symptoms are exactly that of failing module....

+2

I've never hear of an o2 sensor leaving you stranded either.
It may run like crap but shouldn't prevent it from starting or running.

I bought an '87 GN last year from someone who previously suffered the same symptoms you mentioned.

Six months prior to me buying it he replaced the IgnModule/Coil and he said the problem went away.
I've had it six months now, no incidents.

-Tim

 

[bmackinnon]

Best choice 'de-facto' is indeed the oem AC Delco...

HIGHWAY STARS - nos4gn Buick Grand National Parts 1986 1987 Buick Turbo Regal T-Type GNX parts

bottom of the page, Glen offers 2 choices... an 'economy' module for $84 or the OEM AC Delco for $219... I would bite the bullet and spring for the OEM if it were me... but many get away with the repops.... if you go el-cheapo, you may be better off to get one from the local big-box store....as you 'might' be able to swap the possibly defective 'new' part a time or two until you get a good one.... results have been mixed on the aftermarket modules... your choice.. :wink:

OR... you might get lucky in the for sale section here, and find a used 'known good' module for a decent price, say $50-$75... you take your chances there too, though....

OR....just do an extended 'borrow' of Drifter's.... and pay him bottom dollar for it later... he ain't never gonna' get that thing done...

 

[TurboTJT]

+2

I've never heard of an o2 sensor leaving you stranded either.
It may run like crap but shouldn't prevent it from starting or running.

I bought an '87 GN last year from someone who previously suffered the same symptoms you mentioned.

Six months prior to me buying it he replaced the IgnModule/Coil and he said the problem went away.
I've had it six months now, no incidents.

-Tim

I guess I spoke too soon! Had my first encounter with the above symptoms. I thought the previous owner cured it with a new IgnModule/CoilPack. Don't know if he used ACDelco or brand 'X' yet. I guesstimate the combo to be 1 yr old with less than 500 miles.
Could it have gone bad again, that soon?

Seems not to long ago one of our club members had his oem MAF go out.
I remember he had idle/driving problems before he replaced it.
BTW I'm also running the oem MAF, my GN has 34K orig miles.

Help! I'm needing help now too.
-Tim

 

[robertmee]

I've got a new AC Delco IM on the way....Still looking for a reasonable coil pack (nos4gn out of stock). DailyDrifter is going to loan me his IM/CP and I plan to install this weekend, so I'll let you know if it changes anything. I'm running the aftermarket MAF/translator.

 

[TurboTJT]

I've got a new AC Delco IM on the way....Still looking for a reasonable coil pack (nos4gn out of stock). DailyDrifter is going to loan me his IM/CP and I plan to install this weekend, so I'll let you know if it changes anything. I'm running the aftermarket MAF/translator.

My symptoms are very similiar to yours & what the previous owner stated was happening a year ago. I stalled out twice on the road last night, wouldn't start (or at least keep running). So each time I waited ten minutes and limped it home. Of course, when I get home, it starts & idles normal.

Two things worth noting, it was 85 degrees out and had been driving over 30 minutes prior. Also noticed a lack of snap, seemed to be driving sluggish before the episode.

I've got a caspers coil/ign tester. I'll test it first and let you know the results.
I've got a "known good" backup GM ign/coil setup. Depending on how it tests I may swap out the setups this weekend.
-Tim