79 928 - loss of power after driving on highway

[Rupesy]

Once I get it fired up, warmed up and drive down the road I get a glimpse of what this car can do. However, once I hit the highway and drive for about 5 -10 minutes, it won't go over 3-4k rpm and starts to stutter. If I shut it off for about five minutes or just let it sit and idle for about 5 minutes, I can start it and I get that glimpse again for a couple minles down the road. If I put putting around town, I don't notice it as much but it looses power there too. I have read it is most likely the "CIS". Could it be a vaccum problem as well? I will definitely looking into the CIS since it sat for so long. Just want to make sure I don't miss some other things to check first.

BTW, I just made an order for all the standard stuff like oil, gas, air filters, belts, spark plugs, etc. Will be placing orders for more stuff in next few weeks as I learn more about the car.

thanks!

[orange9mm]

since it appears to be engine temperature related and poor performance i would first look into the temperature related CIS components like the "Warm Up Regulator", "Thermo Time Switch", and "Aux. Air Regulator" to make sure they are performing properly. if they are then perhaps you have a bad air/fuel mixture. something too rich and when the aux air regulator closes you don't have enough air. just some thoughts.

[Danglerb]

Also reminds me a bit of a clogged fuel filter.

[Rupesy]

Thanks for the info. I have the fuel filter on order. Since it sat for two years and it was an empty tank (some people just suck) I figured there might be some issues with the fuel system. Here a bit more information. The tank was empty Friday so I filled up before driving home (2nd fill up since I bought it and I had the exact same problem on the first fill up). I have a 30 mile hike on the highway. After about 5-10 minutes, it started to lose power. After about twenty minutes, I had to pull over I was going so slow until I hit the peak of the hill and started to go downhill. Manage to make it home. Later that night, hit the highway again for a shorter drive over to the next town and didn't seem nearly as bad. Did the same highway drive this morning and same thing..not too bad. Anyway, Maybe this additional info could help also. Did some other put putting and seemed fine on the short runs.

[DPW928]

It could be fuel or electrical. I suspect the later. The CIS system is linear in that if there is a problem it will normally show up when cold or hot. Also, I doubt it's the fuel filter as the CIS uses more fuel when cold. I do recommend you add a good fuel system cleaner like Berrymans or Techron to clean out the system.

Electronic areas to check are as follows:

All electical fuse, relay and ground contacts
green wire from distributor
contamination or a crack in the distributor cap and rotor
spark plug wires
spark plugs

Happy hunting.

[Rupesy]

Was playing around tonight and found a vacuum line that was not connected. I attached a picture to show where it was disconnected. The other small line goes to the distributor.



The vacuum line splits into two components. One end of the disconnected line goes to a diapgram that is on what looks like a disconnected emmission system. The other tube goes to a component and I don't know what it does. I attached a photo of it.



The end connected to the diaphragm on the emission rail broke so I just plugged it with a screw. I started the enjine and it was more difficult to start than normal. I let it idle for a while and when I touched the gas,the idle started to swing between almost stalling and about 900 rpm. Drove it down the road and the same thing, when I came to a stop it would go through this cylcing with large swing in rpm. It really ran like crap too. So I took it back and pulled the screw out of the line. THe rpm swing decreased, however, it was still cycling and the driving still sucked but was marginally better. Then I came back and pulled the line from the origin on the manifold and I was back to normal. Very small swing in rpm (50-100 pm) and after going down the road it didn't cycle at all. Most of the power was back so it was back to normal for me. Not sure what that device does, but maybe its not working. Also seems like having an open vacuum line would make it run worse, not better. Hope this is a clue to what might be going on.

Thanks

[DPW928]

The item in the bottom picture is the deceleration valve. It has an internal diaphram that could be ruptured and cause the idle wander.

Dennis

[MPDano]

Yes, I would remove both those lines going to the throttle body and plug them there and the single line going to the Decel Valve. Also, plug the line going to the Diverter Valve. Both of mine had ruptured diaphragms and were leaking vacuum big time.

[Danglerb]

Leo I am sure means, disconnect and plug, as a TEST only, replacing the bad part with a good one if found defective.

[MPDano]

Yes, as a test. I was rushing this morning to try and answer the thread before work. Use this as a test and if it works, then replace the defective part (ie: decel or diverter or both)

[Rupesy]

I looked up the part in the PET catalog and it calls it the vacuum limiter. I am assuming the deceleration valve and the vacuum limiter are the same. It looks like it has a connection above an below the butterfly valve. I guess when the valve closes it allow some air to slip by??? Just trying to understand what it does. I will follow the suggestions and post an update.

I also want to give an update on drivability since it was part of the initial post. Today was a good day for temperature extremes. When I woke up, it was 30F and had the usual hard start and drove pretty bad. Didn't matter that the engine was warmed up. Drove consistent at 3k rpm on highway but had no power behind it. Struggled to get through the gears. LAter, Sun came out and shined all day and we got to 60F today. Went out for lunch, fired right up and ran like a champ around town. No stuttering and lots of power. Lots of fun. At the end of the day, it was a little warmer and it ran like a champ all the way home. It was like a totally different car. This is the way I imaging this car to run. Lots of power and no loss of power entire drive home on highway or short trips. It seems like the fuel delivery system is all working fine at higher outside temperatures. Seems like the issue is related to outside temp going into engine. I know there have been a lot of suggestions where to look first. Hope this extra information helps to support working on the cold start valve, WUR or the other air valve.

[MPDano]

Yup, decel and limiter are the same. Plug both connections above and below as I stated for testing of the Decel Valve.

[Danglerb]

When you blast down the road and run the engine up to redline, then take your foot off the accelerator completely the manifold vacuum gets VERY high, so little air per revolution that combustion becomes unreliable and unburned fuel and air head down the exhaust pipe to the glowing cats.

BOOM.

Vacuum limiter, aka fly saucer, bypasses the throttle plate, and when vacuum exceeds some low value it opens to let some air through.

************

Gotta be some temperature sensing part on your car isn't working or isn't hooked up right. Start with the cheapest and replace until OK.

[Rupesy]

OK, I pulled off the decel valve. I can pull a vacuum from the one side (where small vacuum line goes) and it will hold so I don't think its leaking through the diaphragm. On the opposite side with two connections, I can't blow through it when there is no vacuum on the back side. I haven't figured out a way to test if it opens when there is vacuum. I guess I will need a second person unless someone has a better idea. Is this valve normally closed when there is little or no vacuum? I am assuming this valve should open when there is a significant amount of vacuum right?

[Danglerb]

Sounds OK to me, make sure its hooked up correctly.

[Rupesy]

Well finally got everything back together. Replaced all the vacuum lines, and a few small cooland lines while I was in there. Hooked up the decel valve properly and hooked up the second vacuum line to the distributor. Took the usual time to get fired up since I still have an issue with the CSV circuit. Once fired up, the engine cycled heavily and seemed worse than when I started but sort of expected this. I noticed when I held the accelerator down a little the idle would straighten out. I remembered a small set screw on the front of the throttle body. Messed with that until I got it to idle properly. I guess the PO adjusted that to get it to run right instead of fixing the real issues. The only issue is that I can't set it below 1000 rpm (dash gage) or it starts to cycle again. I'm not sure how accurate the gauge is anyway so I will check it with a different meter. I think I should be able to set it lower without cycling but I will take this for now. The engine actually runs extrememly well now. This is also because I fixed the spark plug issue on the #1 cylinder. So all day long zoom,zoom,zoom like it was a new vehicle. I had absolutely no problems what so ever all day long.

UNTIL.......I filled the tank up at the gas station. I topped it off like I normally do. Got on the highway and it just started to go downhill popping and sputtering and I had to pull over about six times. The only thing that seemed to work was to shut it off and sit for a while. I would then be able to get down the road a little further. This is by far the worse that it has run. The ride is a 1/2 hour and the farther I went the better it ran. Still not working great at this time.

I have noticed that every time I fill up the gas tank this same thing happens. It hasn't happened this bad before but generaly happens the same way. I noticed that the tank gage said 3/4 after I filled up and as I was driving it was actually going towards full. That happened the last time I filled up too. What am I missing here? What should I be looking for? Maybe I shouldn't be topping off full like I am??????

[DPW928]

Try adjusting the idle with the large set screw in front of the throttlebody. It sounds like there is crud in the tank that blocks the strainer whenever you fill the tank. Might try removing and cleaning or replacing the strainer/internal pump.

[785sp]

Ditto, Dennis. The simplest explaination is that after 30 years, there's two inches of crud in the bottom of the tank. Remove the tank straps, disconnect the fuel pump apparatus, unscrew the plug/filter, drain completely, re-plug temporarily, pour in a bottle of Marvel or whatever, let it sit, drain again, reach up in there with your finger and a rag and remove as much of the crud as you can, reassemble with a new plug/filter, and you're probably good to go. It sounds like your fuel pumps are still functioning; good fortune there.

The 3/4 full issue is another story and a different problem; lots of posts on that.

[Rupesy]

Had that in the back of my mind, but just didn't want to entertain it for obvious reasons! Makes sense. Ususally don't have an issue the next morning. Maybe it has time to settle. Will try to schedule something when tank is closer to empty....which won't take long!

[MPDano]

Yeah, I ended taking mine completely out with the advice of others. Trust me, I fought this one but in the long run, it was the absolute right decision. I brought mine to a Radiator shop and they acid washed it for I think $60. Worth every cent!