97 D1 Fluttering/Hesitation at highway speeds

1of40

Active member
Oct 23, 2017
25
2
Va
Trying to help my kid get ready to shove off to school. His D1 is giving a fluttering/hesitation like it might want to stall. It has only done this at operating temps and hey speed. Btw engine temps are about 210 at the tstat housing on a 90+ degree day like today. We did a test drive with heat in full to see if it also happens when a bit cooler and didn’t get it to repeat. We’re gonna try again at night with cool engine and cooler temps.

Fuel pump is new so thought we might start with new plugs wires bc the temps at the manifold vary between 300 and 450 depending on which port we’re shooting. My guess faulty plugs could be ramping up the heat. Fan and coolant seem okay.

Any other recommendations?
 

robbyb20

Well-known member
Oct 2, 2019
70
12
Seattle
Do you have an ultraguage or any OBDII reader handy? You may not have a CEL but there could be codes stored. I am going thru the same thing and think I have it sorted out.

If you hook up an ultraguage or OBDII and connect it to your phone or other device, make it so that it will show the MPH in real time. Also run a check on any pending/engine codes.

What I was seeing was that my VSS (vehicle speed sensor) was jumping wildly about. Going from 20-90-50-etc until it finally hit about 110mph and higher. At that point, then engine would buckle and throw a code at me but not illuminate the CEL.

From what I can understand, once you hit those high speeds, the ECU tells the engine to cut power to the spark plugs and that causes the lull and accompanying misfires. You may also see these codes, P0300, P1316 or P0301 thru P0307 once you hook up a reader.

You can also test this by unhooking the VSS under the car and driving at highway speeds. its next to the transfer box by the brake drum. You will lose your speedometer but it wont be sending data the ECU to cut the power to spark plugs. If you have no issues driving on the HW with this test, then thats a good indication its your VSS.

Im sure it could be a myriad of other things but thats what it was with mine and it sounds really similar.
 

1of40

Active member
Oct 23, 2017
25
2
Va
Do you have an ultraguage or any OBDII reader handy? You may not have a CEL but there could be codes stored. I am going thru the same thing and think I have it sorted out.

If you hook up an ultraguage or OBDII and connect it to your phone or other device, make it so that it will show the MPH in real time. Also run a check on any pending/engine codes.

What I was seeing was that my VSS (vehicle speed sensor) was jumping wildly about. Going from 20-90-50-etc until it finally hit about 110mph and higher. At that point, then engine would buckle and throw a code at me but not illuminate the CEL.

From what I can understand, once you hit those high speeds, the ECU tells the engine to cut power to the spark plugs and that causes the lull and accompanying misfires. You may also see these codes, P0300, P1316 or P0301 thru P0307 once you hook up a reader.

You can also test this by unhooking the VSS under the car and driving at highway speeds. its next to the transfer box by the brake drum. You will lose your speedometer but it wont be sending data the ECU to cut the power to spark plugs. If you have no issues driving on the HW with this test, then thats a good indication its your VSS.

Im sure it could be a myriad of other things but thats what it was with mine and it sounds really similar.
Excellent! We’ll try these suggestions and get back with results. Many thanks.
 

discostew

Well-known member
Sep 14, 2010
5,561
255
Northern Illinois
I was thinking VSS too. It won't store a code because of the way it fails. Because of the way the signal breaks down the ECM sees it as a bunch of extra pulses or MPH. So if it is the VSS, the truck thinks its going like 120 or more and starts to shut off a bank of injectors. Since the computer is doing this its self, it doesn't store a misfire fault.
 
  • Like
Reactions: robbyb20

discostew

Well-known member
Sep 14, 2010
5,561
255
Northern Illinois
The vehicle won't ever shut off the spark plugs, only the injector. You wouldn't want to send fuel with no chance of burning it. And it's usually half the cylinders.