Search



Go Back   MustangForums.com > Ford Mustang Tech > 5.0L (1979-1995) Mustang
Sign in using an external account
Register Forgot Password?
Search

5.0L (1979-1995) Mustang Technical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.

Sponsored by Weld Racing


Welcome to Mustang Forums!
Welcome to Mustang Forums.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, at no cost, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!


Reply
 
 
 
 
Thread Tools Search this Thread
Old 11-30-2009, 01:22 AM   #1
Deadweight70
 
Join Date: Jul 2009
Vehicle: 1990 Ford Mustang
Location: Florida
Posts: 9
Default what would cause these rpm fluctuations?

1990 Mustang 5.0 stock engine...stock fuel pump.... pretty much all stock
Manual transmission

When I am driving the vehicle, at highway speeds the rpms will randomly drop typically from 3000 to 2500 and back up again in rapid succession. This is not happening when the vehicle is under acceleration. I'm just cruising along at 80 or 85 mph and the vehicle starts feeling like it is bucking. I drive about 35 miles one way to work and it tended to occur once or twice during the trip. This problem occurred over the course of a few months. Now it has worsened and it is occurring at speeds less then highway and the frequency has increased. It never occurs under 35mph.

Also if I am accelerating hard it has recently started to duplicate the same symptoms. If I am in third gear giving it full acceleration pushing past 4000 rpms the rpms will plummet to 3500 and back up again very rapidly.

The ignition components on the vehicle are newer. Spark plugs, wires, and coil are new. They were changed after the problem developed. The problem has worsened since. I changed the spark plugs and wires as maintenance and someone else recommended the coil as a possible fix so I changed that to no avail.

I am assuming it is fuel related.

the check engine light is not on

any ideas or advice would be appreciated. Thanks!
This ad is not displayed to registered or logged-in members.
Register your free account today and become a member on Mustang Forums!
Deadweight70 is offline   Reply With Quote
Old 11-30-2009, 01:54 AM   #2
mattdel
6th Gear Member
 
mattdel's Avatar
 
Join Date: Nov 2006
Vehicle: 1986 LX, 87 GT Hatch, 89 GT Vert
Location: Spfld, MA
Posts: 9,240
Send a message via AIM to mattdel
Default

first step would be determining whether you're losing fuel or spark, which is sometimes hard to do when the problem is only recreatable under driving conditions.

my first step here would be checking/replacing the fuel filter. after that, if the problem still exists, buy a fuel pressure gauge set, and tape the gauge to the windshield as you drive. when it happens, quickly look at the gauge, and make sure its where it should be.
if it is, its time to move on. at this point, you'd want to check the TFI module and the PIP module, both of which are in direct control of fuel and spark control.

running codes might not be a bad idea, either, as a TPS or MAF issue could cause this as well, but the other ideas are more probable for failure.
__________________
mattdel is offline   Reply With Quote
Old 11-30-2009, 01:50 PM   #3
Deadweight70
 
Join Date: Jul 2009
Vehicle: 1990 Ford Mustang
Location: Florida
Posts: 9
Default

thanks for the info....

gonna change the fuel filter on wednesday and see if that makes any difference...

I wish it replicated the problem at idle on the driveway.... would make it so much easier to figure out.

I'm not looking forward to rigging up a fuel pressure gauge to windshield but I guess my only other choice is the "wait till it finally really breaks and stops running" option that I was so fond of in my youth
Deadweight70 is offline   Reply With Quote
Old 11-30-2009, 03:38 PM   #4
mjr46
Wal-mart lube/tire tech
 
mjr46's Avatar
 
Join Date: Jun 2007
Vehicle: 85 mustang
Location: West Virginia
Posts: 29,770
Default

could be a tfi or stator issue in the distributor
__________________

Quote:
Originally Posted by dudegirl View Post
Be polite and cordial when you talk to them, as pissing off the guys that can help you is seldom a good idea.
mjr46 is offline   Reply With Quote
Old 11-30-2009, 05:12 PM   #5
Deadweight70
 
Join Date: Jul 2009
Vehicle: 1990 Ford Mustang
Location: Florida
Posts: 9
Default

alrightie..... hope i solved the problem.... be prepared to be amazed at my ignorance!

so after reading the couple suggestions here and doing some research I decided to go out to the car to make sure everything on the distributor was plugged in nicely. As i pressed the plug in, I noticed the whole distributor shifted. It was quite loose.

I have no idea how to time a car so I just turned the distributor till the car sounded like it was running smooth and snugged the bolt down.

now I've had what I thought was an unrelated problem that someone else told me might have to do with timing. If I drive the car for thirty or fourty minutes.... stop somewhere... like the gas station, where the car is off only for a few minutes and then I try to restart the car it has trouble starting.

so I am hoping against hope that I solved both problems. Likely?
Deadweight70 is offline   Reply With Quote
Old 11-30-2009, 05:33 PM   #6
mattdel
6th Gear Member
 
mattdel's Avatar
 
Join Date: Nov 2006
Vehicle: 1986 LX, 87 GT Hatch, 89 GT Vert
Location: Spfld, MA
Posts: 9,240
Send a message via AIM to mattdel
Default

possibly. if not, the first thing to diag would be the TFI module, which is bolted to the front of the distributor.

and get it timed correctly. you run the risk of doing serious damage with the timing not right.
__________________
mattdel is offline   Reply With Quote
Old 11-30-2009, 06:23 PM   #7
lxman1
4th Gear Member
 
Join Date: Jan 2008
Vehicle: 1988 Ford LX hatch
Location: Louisville, Ky.
Posts: 1,319
Default

If you mean that the starter drags when trying to restart, then you need to turn the distributor back counter clockwise a bit. The timing is too high.
__________________
1988 5.0 WC T5 LX HB (Original Owner)
Oxford white
Saddlebeige interior.
TF Track Heat
BBK 70MM TB
C&L 73MM MAF
24lb injectors
Trickflow TW heads with upgraded dual springs
TrickFlow stage 1 cam
Magnaflow Cat-back
lxman1 is offline   Reply With Quote
Old 11-30-2009, 06:55 PM   #8
mjr46
Wal-mart lube/tire tech
 
mjr46's Avatar
 
Join Date: Jun 2007
Vehicle: 85 mustang
Location: West Virginia
Posts: 29,770
Default

define trouble starting??
__________________

Quote:
Originally Posted by dudegirl View Post
Be polite and cordial when you talk to them, as pissing off the guys that can help you is seldom a good idea.
mjr46 is offline   Reply With Quote
Old 11-30-2009, 07:20 PM   #9
Deadweight70
 
Join Date: Jul 2009
Vehicle: 1990 Ford Mustang
Location: Florida
Posts: 9
Default

define trouble starting-----

basically it feels like the battery is very weak.

All of the electrical components in the vehicle work just fine. Only one time it wouldn't start on the first try. I let the car sit for about ten minutes and it refired fine.
Deadweight70 is offline   Reply With Quote
Old 11-30-2009, 07:31 PM   #10
aode08
5th Gear Member
 
Join Date: Oct 2005
Vehicle: 91 Mustang GT
Location: Ohio USA
Posts: 4,614
Default

have the dealer set your timing
__________________



347 , 11:1 Compression,Knife Edged Lightened Crank,AFR 185,Ported Trick-flow Intake,Ford X-CAM,Under-drive pulleys,aluminum drive-shaft, 3:73 gears,Fender Cold Air Intake,Long tube headers,MSD Digital Ignition System
Lightened chassis.

451HP 439 FtLbTrq. mid 10 second car !
aode08 is offline   Reply With Quote
Old 11-30-2009, 07:31 PM
MustangForums
Ford Mustang




Paid Advertisement

 
 
 
Reply

Tags
1990, 2005, 50, cuae, drop, ford, gauge, minute, motor, mustang, readings, rpm, rpms, runns, stops

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Forum Jump

Advertising

Featured Sponsors
Vendor Directory
New Sponsors
All times are GMT -5. The time now is 02:01 PM.

© Internet Brands, Inc.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

This forum is owned and operated by Internet Brands, Inc., a Delaware corporation. It is not authorized or endorsed by the Ford Motor Company and is not affiliated with the Ford Motor Company or its related companies in any way. Ford® is a registered trademark of the Ford Motor Company
Emails Backup