70 MPH bucking problem

[yrlessbob]

Hello all,

I have a pure stock 87 GN 54,000 mi with a couple of issues. At 70 mph and above it will start bucking and dying out like running out of fuel, slow down to 65 and runs like a champ. Under acceleration no problems, shifted out of OD to change RPM, problem still there. Doesn't do all the time though. First 20 miles today no problem then started and continued for the next 80. On way home first 20 just fine then started acting up again but did clear up when ran into rain but problem back 10 miles past rain. In the past seems like if acting up and car gets shut off a couple of times it goes away for awhile but eventually comes back but this is always in dry conditions. Any ideas???

Problem two - Damn cruise control works great for a time then out of the blue it will start accelerating going to WOT. I can hit the coast and it will stop and when back down to speed it will hold it for a short time then it slow down like I disengaged the cruise. Had a similar problem on a 79 Monte but had a bad ground. I have checked ground and made sure had good contact. I am thinking bad switch but looking for second opinion.

On a side note I did notice today on way home that when cruise started acting up sped up and bucking problem gone. Bucking problem came back, slowed down and cruise was working fine. Not that one has anything to do with the other but just an observation I made.

Thanks in advance.

Bob

 

[Ssg Ken]

Lack of gas

I had this in my 1960 Pontiac with tri power. I could go about 75 on the center carb, part throttle, and feel the stop that opened the other carbs. If I tried a sustained speed with the 3 carbs, it stalled out, gas starved. Ran fine for a quarter mile. It needed a bigger fuel pump.

I would like to know if you have replaced the fuel filter lately?

 

[yrlessbob]

I replaced it about a year ago right after I bought the car and old filter seemed fine. Have 28 lbs of fuel pressure on the fuel rail and I can't duplicate the problem in the shop.

 

[DailyDrifter]

FP seems low. I was going to say lock-up clutch constantly engaging and disengaging, or trans trying to downshift.

 

[yrlessbob]

I was wondering if that was correct FP on the rail. Has stock 28 lb injectors and have been wondering about regulator, fuel pump, or bad injectors. Besides the bucking it will also die out for one to two seconds like running out of fuel.

 

[DailyDrifter]

It could be running out of fuel, clogged fuel filter, clogged fuel sock. Is the original pump still in the tank? Do you have any sort of scan tool to read the o2's at 70?

 

[yrlessbob]

As far as I know it is the original pump. No I don't have a scan tool but a friend may have one and will check. Trying to not spend money where not needed (this new invention called a budget lol) and appreciate your input. Forgot about the sock on pump.

 

[SPOOLFOOL2]

Yo Bob,

At NO time should you be at only 28 lbs of fuel at the fuel rail. If this is true, then you are very lean. The only reson that you have not hurt the motor is that your at a low power level (we can fix that too ). What area do you live in? This is very simple and cheap stuff to fix. Don't flog the thing till you get it fixed. It aint that hard.

Mike B.

 

[yrlessbob]

Live in Northwest Iowa. All ears to cheap fun

 

[WSLN 6]

A fuel fump upgrade is not optional, it is mandatory. Hotwired pump and scantool should be the first purchase. Either that or take all your floor mats and insert them under your gas pedal to limit yourself to half throttle....:biggrin:

 

[QUIP'6']

I replaced it about a year ago right after I bought the car and old filter seemed fine. Have 28 lbs of fuel pressure on the fuel rail and I can't duplicate the problem in the shop.

put it at 43 psi with the vaccum line off

 

[SPOOLFOOL2]

Live in Northwest Iowa. All ears to cheap fun

Introduce your self in the Mid West section. Hook up with the locals. Lots of trading of knowledge and used parts going on. Ask questions and use the search. Lots of good info and good people here.

And, go get a XP fuel pump. Most of the venders here have them.

Mike B.

 

[thewoodieb]

Problem 2

Problem 2 was actually pretty common, I seen it on a tsb the other day, apparently its a small bushing on the cruise linkage that would cause the car to hit wot under the cruise option. This was a recalled part. It was on most g-bodies with cruise. I can find out more about it if you would like, the bucking could be detent, lean, or lockup. Unplug your lockup see if it still does it. If no keep it unplugged or replace the lockup solenoid.

 

[yrlessbob]

Finally able to get back to this.....life took over fun I replaced fuel pump with AC Delco replacement and now have 38 lbs fuel pressure w/vacuum off and problem still there. Bypassed the lock up problem still there. Now have a scan master :smile: but of course car ran fine but the O2 levels varried from the 80's on the low end to the 670's on the high end jumping up to the 740's once in a great while. Is it normal to have the O2 levels varying that much or should it be a steady reading?? Could the regulator be bad? Am considering swapping out pump again with recommended one.

I am also interested in what you might be able find on that bushing for the cruise.

Thanks to everyone for all your help, advice, and ideas.

 

[pacecarta]

check the exhaust for for obstruction (cat converter melted)
.


the o2s at closed loop will jump up and down ... for an idea of how the fuel supply is you need a wot 3rd gear reading when the car is in open loop PE mode you want to see 780 or better , if not lower boost or fix the fuel issue

the doesnt do it all the time sounds like ign module

as for the post of a 'bushing on cruise linkage' , ive never heard of it . The cruise on out cars is a simple cable and a vacuum bellows to pull the cable , and unless you have cruise on it couldn't have any effect on the cars operation