Topic: Vehicle bogs upon acceleration BUT only during the summer

[Stringer]

Okay, I have have an interesting problem that I can not figure out.  I have a 1979 Tran Am with an Olds 403.

Long story short.....everything is new or within a couple years old (gas tank, carb, fuel pump, distributor, ignition coil, valve job, etc, etc, etc.).

The problem....the vehicle bogs upon acceleration BUT ONLY WHEN THE OUTSIDE TEMPERATURE IS OVER 90 DEGREES!!!!? And only after the vehicle is good and warm.  It exhibits NO symptoms of vapor lock. It is not hard to start, there is no strong odder of fuel, etc.  The only problem is that the vehicle bogs when taking off from a stop, primarily after idling for about a minute, and after 15-20 minutes into the drive.

The really strange part is.....it only does it during the summer. I can drive the car all fall, winter, and spring and the car performs great (I live in Phoenix AZ)....but when the outside temperature is over 90 degrees I experience intermittent bogging when accelerating from a stop.

The vehicle has done it every summer since I got it from a friend in 2014.  The car is a daily driver and I am restoring as I go. To date I have over $12K in the restore and each summer I try keep having this same issue.

I am thinking maybe heat soak?  I even replaced the ignition coil a couple days ago and still the same problem.

Has anyone else experienced anything like this?

[Stringer]

Looks like I have everybody stumped.  Over 120 views and no one has any suggestions. 

Okay some more on the back story.....

I bought the car from a buddy of mine I have known for 20 yrs.  He had the car since 1986. It was garage kept most of its life but has spent a few years baking outside in the sun on and off during that time.  I have choosing to restore it as stock as possible. To date I have rebuilt the suspension, steering, braking, and fuel systems. I have had the heads professional redone and I have taken care of other engine work.  I have 2 1/2" dual exhaust coming off the stock manifolds with no catalytic converters. And I have redone the interior (with seats and a couple odds-n-ends left to go).

I drive the vehicle to work daily, putting about 200/week on it - both in town and freeway driving. The car actually does quite well. It goes, it stops, it shifts well (it's an automatic), and it maintains good temperature through all types of driving (freeway, idling in traffic, etc.) even on days over 110 degrees. All-n-all it is a good solid vehicle. I just keep having the issue as described above.

It appears to have gotten better as I have gone along, but I have never really been able to nail it down.  It feels as if it is out of fuel when it occurs. The vehicle just falls flat on its face.  You can let off the accelerator and it will idle, you press the accelerator again and it immediately bogs down.  If you keep the pedal pressed there will be a few deep bogs between light surges. Eventually it will recover and drive as if nothing has happened. It happens most under moderate to hard acceleration. The harder you accelerate the worse it is. Under light acceleration it generally does not occur, but I do feel occasional light hiccups as if it wants to bog down as I slowly accelerate. 

But, as stated in my original post........It only happens after driving the vehicle 20 about minutes and only on days over 90 degrees. I do not experience this problem during the cooler parts of the year.

[2pfl]

I’m no expert but sounds like it’s vapor locking

[Nexus]

I would also double check the vacuum lines and make sure that none are cracked and they are going where they are supposed to go. I remember reading a similar post somewhere and the problem was in those lines and them not being put back to the correct locations.

Just a thought

[Stringer]

Thank you both for replying. I really have been racking my brain on this one. I too had thought bout vapor lock but the vehicle exhibits no other symptoms of vapor lock.  Over the years I have had various different configurations so to speak ..... i.e. I am replacing parts as I go. It is a daily driver that I am restoring. 

I have also tried different "tricks" along the way like insulating fuel lines, etc. All to no avail.  I have keep it stock and everything is as it should be from the factory. I am not aware of any vacuum leaks.  Like I said....the car runs great until it gets over 90.

That said......I did replace the gas cap just as the temperatures started to soar this year (as the valve was stuck and was not allowing air into the tank). Since then I have noticed that the bog does not start until the temperature is closer to 100 degrees.  Strange that a gas cap would do that; but it did. 

I am starting to think it is something along related to fuel pressure; but I know my pump is good.  It is the second one I have put on it since I have had the vehicle. The vehicle has always has had the bogging issue since I got it.  It is also the second carburetor I have put on it as well.

When it first started, it would do it when the outside temperature was about 90. New pump, carb, valve job, ignition coil....only made small amounts of difference.......but what has made the biggest difference is the gas cap....seriously, the gas cap.

Any other suggestions because I am baffled at this point.   I would rather not put an electric fuel pump on it. From my line of thinking...I shouldn't have to. The engineers didn't design it to do this, else it would have been doing it since 79....only difference is the ethanol in fuel these days. That could be the issue. I just don't know.

[Nexus]

Have you tried driving it without the gas cap to see what does with full venting in?

[Stringer]

Yes. By late April Phoenix is about 85 degrees.  We had a few hot days (95-98) and I experienced the bogging. Out of curiosity I checked the gas cap and it was bad. I left the cap lose for a couple weeks.  Temps went low, then back to normal, then by mid May we had our first days around  95-100.  I did not experience the problem until the temp was about 100.  Now temps are 106-111. 

I can do a moderate acceleration at 100 and the hotter the temp the more sensitive it is.  I have also experienced "light bogs" when traveling 35 and accelerating moderately to pass during street traffic (when temps are about 105)

I drive the car to work in the morning (temps 70-83) and not a problem at all. I drive coming home (temps 106-114) I experience the issue.

* Good news - I should have the seats done in a few weeks; materials have been ordered.

[Wallington]

Looks neat. Is that the engine harness wrapped up and routed across the heads?

[Nexus]

It just clued in to me why your having issues!!

You must own a Canadian car!!!

The temps never get that hot where I live so I think you should just give me the car for its sake, I will give a good home where it will never have to experience heat like that again

 

[Stringer]

Thanks. Yes, the wires to the alternator and water oil pressure gage are in the loom. They are in a heat resistant sleave draped across the head. In 4 years I have never had a problem with it. The wires are not even hard or anything.

The car made in Van Ives California and was sold in Oklahoma in 1979. It made its way to Las Vegas were my friend drove it at the age of 16. It was more or less parked in 1992 and was only driven occasionally until I bought it in 2014. Last year I even found the  actual dealer plate for the vehicle....it had been trapped in the trunk wedged between the inner and outer panel....the paint is half pealing but the 1980 registration sticker is bright and clear.  What a find! LOL

[Nexus]

Right on!!

I've been trying to come up with some ideas for you but I'm quickly runing into dead ends...wish I could be more helpful

[Stringer]

**UPDATE**

Believing my problem to be fuel related I sought out a high performance mechanical pump for my Olds 403. The specs for the one I had was 30/gph @ 5.5psi. The one I replaced with was a Spectra with a flow rate of 45/gph @ 7psi.  I have been driving it the last few days with today being the hottest at 108 degrees. The vehicle feels much stronger all the way around. Weather it is cool outside or hot, it feels better stronger during the acceleration.  It is much better than before.

That being said...... I am still experiencing a little flutter under hard acceleration. I suppose I can live with that because for normal (and moderate) acceleration it would appear that the higher flow rate and psi of the pump is keeping up.

Anyone know of an even higher flow rate pump?

[Nexus]

Glad some of the issues have been cleared up...that flutter on the WOT I'm thinking is just a tweak in the jetting but more experienced people might be able to give you better insight.