1990 3.3 Plymouth GVger bucking at 20-35

[JohnnyOH]

Had tranny and engine computers out yesterday.
Replaced engine computer with one from my favorite junkyard.
Test run under initial engine start was excellent.

I turned engine off for 5 minutes. After that, the engine went back to its old ways: it bucks under a load (like up a grade) from about 20 thru 35 MPH.
This has been growing progressively worse over a year. Last fall, it got really bad so I took the van off the road till now.

Based on yesterday's experience, we have ruled out a bad valve and bad compression, since it accelerated from 0 to 65 MPH normally for the first time in a year. TBI was obviously working right during that intitial run.

What does this test point to for the next tests?
Any ideas welcome, of course.

My brother suggested tranny is not shifting right, so I will make a test run in 2nd gear.

Also, I can pull the wires from each computer in succession and look for similar behavior to replicate. That might point to one computer/ system or the other.

I am a vet on my computer forum, but a newbie here. Thanks for your responses and interest in motors!

ps Have replaced many items: fuel pump, electronic ignition block, wires, plugs, fuel filters, air filter, a sensor/sender (forget the name, but it cost $65 USD. Shop computer read out indicated weak fuel pump and that is all. Plugs are all nicely tanned.
All injectors have resistance test in the same range.

JohnnyOH

[JohnnyOH]

Had tranny computer un-wired for 24 hours.
Ran the 2nd gear test (and 1st gear test to cover things nicely) and the tranny computer test mentioned above.
Engine bucks as described above.

Am concluding it is not due to tranny computer or tranny shifting problem.

Removed wires from engine computer and will try that test (referred to above) tromorrow. If the engine works until after I shut it off for 5 minutes and re-start it, I will post it here. Then we can try to figure out what that means.

If the successful run above does not replicate, the van might be headed for my favorite junkyard.

I hear the 1990 3.3 was a junker from the start, anyway.
I just got in on the tail end of the misery.

[caravandrvr]

Put a multitester on your battery and idle the van for about 20-30 minutes with everything electrical on (heater blower, headlights on high, 4-way flashers, etc). See if your voltage drops steadly over the course of that time. I suspect you've got a flakey alternator. This test will detrmine if that is the case. Seen this more then once.

[JohnnyOH]

Re-attached the engine computer and it still bucks.
Removed battery cable and will try a run tomorrow.

Ran the 30 minute test. 14 to 13 and ending at 12 volts. No charging system problems evident.

In need of more ideas. Thanks for that one. Every contribution is appreciated.

[caravandrvr]

Originally Posted by JohnnyOH

Ran the 30 minute test. 14 to 13 and ending at 12 volts. No charging system problems evident.

I would think it shouldn't drop to 12-volts while idling with all that stuff running. I'll bet if you let it run longer it would probably stall out. (Note: If you do this you’ll need a battery charger to get it started again though) Can't say for absolute certainty from here but I think you may have found your problem.

With the 4-ways flashing you'll get a ~.4 volt fluctuation, but the voltage should hold fairly steady over time in the range with the flasher fluctuation. If the voltage drops over time then I’d say there is a problem. If the car drops even for a second below 12 volts it will stall. To have it idling and being that close to 12 is not good. It should be charging at around at least ~13V at all times while running, especially with that much of a load. I think even my worthless Haynes manuel states it should be charging 14-15 volts. I’ve found that it is usually around 13.4-13.7 though.

I’ve seen this intermittent bucking misdiagnosed in every way possible. It’s a tough one, but if you think about it with all the electronic gizmos on these engines they need a good firm clean source of power. If you haven’t changed your alternator in the last 50,000 miles I’d say it is a good bet.

I’ve also got a concern with that diagnosis of a “weak” fuel pump. Not sure what that means – low pressure? If that’s the case it could be a problem with fuel delivery. But what do they mean by weak? I’d say it is either giving enough pressure or it isn’t. Of course, if your alternator voltage is not up to snuff that will impact on fuel delivery via an electric fuel pump. Think about how a processor or hard drive would malfunction with intermittent low voltage drops…. (BTW-know a source for a Clevo laptop mainboard?)

I’m still suspecting the alternator at this point though, but the symptoms you describe could easily be a fuel problem as well. Since you’ve already changed your pump & fuel filters I’m thinking you’re all set there. Maybe try that voltage test a little longer to see if you drop below 12 volts and stall, at least you won’t shell out anymore $$’s for that test (except for the gas-ouch!), and if it does stall you’ll know for sure that’s the problem.

One more thing to check, did you ever do the key dance to check the codes? Sometimes when this happens it will throw a code 41, but many times it won’t. If your check engine light comes on when it is bucking then it is certainly throwing a code. Unfortunately when you disconnect the battery it clears the codes though, so if it does come on again check the codes stored. To check for codes have the car off and turn the key quickly like this without turning it to start:

On-off-on-off-on

Then watch the check engine light for flashing and count the flashes. You’ll get a series of 2 digit codes that end with 5-5. Since you just had the battery disconnected you should at least get a code for that.

[JohnnyOH]

I admire your patience and tenacity.
Reminds me of me on funky computers.
I do not have the board you referred to. Just a Dell laptop in use. Other three are desktops. No spare MOBOS.

Did nothing on van today.
I can certainly replace the alternator myself. Just did the 88 Caravan.
The 3.3 has 130,000 miles, so the alt might be ready for the trash, as you say.

Voltage is very important for a computer, too, so I am down with your obsveration.

Will do the key dance. I have a code book around here someplace.

Have already installed new fuel pump and filters, as I said. Commercial computer read-out said weak fuel pump and that was all, so naturally we thought the problem was in it.

Still have the metal flooring ripped back for access. Makes a great bed for robots. LOL

Will get back to it tomorow.

Thanks, Tenacious!

[caravandrvr]

If you get the codes and can't find your book just post them here and I'll look them up - I've got the actual list from Dodge. Put the year/make/model/and engine size in the same post with the codes to make the reply easier.

At the mileage you've stated, if that alternator hasn't ever been changed I'd say that's another sign. I'd still do a longer idle test though to be sure so you don't waste the dollars on another repair that won't actually fix it, but I'm getting pretty convinced. If that voltage drops over time I'd say that's the sign. If it throws a code, even better.

Good luck, and keep us posted.

[caravandrvr]

Oh yeah, and one more thing...

Not sure what engine you had on the other alt you changed, but on that 3.3 it's a real PITA!

[JohnnyOH]

Other vans are 88 Dodge Grand Caravan 3.0 and 87 Plymouth Vger 2.6.

The 88 had the belt and alternator changed by me. I bought the arm for releasing the idler tension.

If the 3.3 is a bear to change, I have a pro mechanic.

Big snow deal here this weekend, so worked on 5 indoor computers (not on the van!).

[JohnnyOH]

key dance was: 1 2 pause 5 5.

This was after re-connecting battery negative cable and then ALSO after taking a short spin.

Have not located codes book.

The spin was not conclusive about bucking.
The engine stalled three times while warming up. I had wanted a clean ride after re-connecting the battery... to replicate when I had both computers out last week.

I did get up a hill pretty easily, but there might have been some bucking.

I did not have wallet with me, the garage door was open and there is no exhaust system, so I hesitated to give it a longer test run.

But I got the codes, so that is a start.
year: 1990
make: Plymouth
model: Grand Voyager
engine: 3.3 litre 6 banger
tranny: automatic

[caravandrvr]

The 12-55 codes are insignificant.
12=something about the battery being disconected recently
55=end of codes

But when you disconnect the battery it clears the codes anyway.

[JohnnyOH]

Checked voltages at batteries on my 3 vans during initial idle:
14.8 volts 2.6 engine (it idles at a higher RPM by the way)
13.8 volts 3.0 engine
13.8 volts 3.3 engine

This works for me. I don't understand the logic of running the test you mentioned, or how it relates to the engine action we are talking about.

If the battery was not charging properly on the 3.3, that would be a different case.

Of course, I don't know what you know about it, so please enlighten me.

The 3.3 engine exhibits that strange behavior on the first hill, and also anytime later on. It is not variable in relation to elapsed time running.

[caravandrvr]

Originally Posted by JohnnyOH

Checked voltages at batteries on my 3 vans during initial idle:
14.8 volts 2.6 engine (it idles at a higher RPM by the way)
13.8 volts 3.0 engine
13.8 volts 3.3 engine

14.8 sounds high, but 13.8 on the 3.3 is just about right - but is that with a load?

Originally Posted by JohnnyOH

This works for me. I don't understand the logic of running the test you mentioned, or how it relates to the engine action we are talking about.

If the battery were not charging properly on the 3.3, that would be a different case.

Since you stated in a previous post that your voltage drops over time all the way to 12-volts that indicates to me that your vehicle is not charging properly since it should never drop that low - especially under a load! 13.8 is about average. You've also claimed that you've already changed out the most obvious issues and are at a loss for solutions. Since this test costs nothing further to conduct I tend to check this when everything else seems to be ok. I have seen failing alternators cause exactly the symptoms you describe, and the only way I've been able to confirm the bad part is by letting the vehicle run under an electrical load for long enough to see if there is a constant dropping of voltage. If there is, then I'd say that's it. If you can idle the thing for 20-30 minutes with all the lights and 4-way flashers going with know drop in voltage then you can rule out the alternator. If, as you stated in a previous post, that the voltage dropped to 12V then you've certainly got an issue with your charging system. At no time while that vehicle is running at idle should that thing go as low as 12 volts.

Originally Posted by JohnnyOH

Of course, I don't know what you know about it, so please enlighten me.

The 3.3 engine exhibits that strange behavior on the first hill, and also anytime later on. It is not variable in relation to elapsed time running.

There is no relation to time running while you are driving because the engine speed is changing, changing the speed of the alternator and all kinds of other sensor inputs. When driving around you are inserting all kinds of variables into the equation/test. However, if you are just sitting there idling with the engine at a constant speed why should the charging voltage drop over time? I've never seen a healthy vehicle parked ideling away with all the lights on just stall out. If your voltage does drop as you have described I'll bet yours will if you let it run long enough. Once that meter flips below 12V it's all over. Although I'm not sitting in front of the vehicle working on it so your mileage may vary... Only trying to help….

[JohnnyOH]

Just ran the 30 minute idle test again.
12.1 volts was the charging rate at 30 minutes.

Will get a junkyard alternator when I can swing it.
Thanks again for your patience and input!

[JGKC9AYC]

Don't know if i'm understanding exactly what you mean about bucking, but I have a 91 GC that, on a cold start, "bucks" down the road.
I put a can of Sea Foam thru the PCV line & it ran great for a few weeks, then back to the same problem.
Hope this helps...if not, perhaps someone knows what the problem is with mine...not that i'm trying to hijack your thread.