Pennock's Fiero Forum
  Technical Discussion & Questions
  Runs fine at low speed but poor acceleration on WOT.

Post New Topic  Post A Reply
Email This Page to Someone! | Printable Version


next newest topic | next oldest topic
Runs fine at low speed but poor acceleration on WOT. by Jim_Martin29
Started on: 07-13-2015 06:59 PM
Replies: 5 (221 views)
Last post by: Patrick on 07-14-2015 02:59 PM
Jim_Martin29
Member
Posts: 702
From: Marina, California, USA
Registered: Jun 2010


Feedback score: N/A
Leave feedback

Rate this member

Report this Post07-13-2015 06:59 PM Click Here to See the Profile for Jim_Martin29Send a Private Message to Jim_Martin29Edit/Delete MessageReply w/QuoteDirect Link to This Post
I need a little help from some of the Fiero system experts here.

On page 6E2-C2-2 of the 1986 service manual, the first paragraph discusses the acceleration mode. It says, “The ECM looks at rapid changes in throttle position and manifold pressure, and provides extra fuel.”

Can you tell me exactly what happens in the acceleration mode? Does it hold the injectors open longer or are they pulsed faster?

I’m having trouble with my ’86 GT. It runs like it has a carburetor without a working accelerator pump.
I have replaced the Throttle Position Sensor (TPS) and the Manifold Absolute Pressure sensor and neither makes a difference. In fact, I think that I have replaced all of the sensors by now.
Here is a link to the previous post;
http://www.fiero.nl/forum/Forum2/HTML/135335.html

Thanks,

------------------
Jim Martin
White '86 GT
Monterey, California

IP: Logged
PFF
System Bot
f85gtron
Member
Posts: 657
From: Augusta, Ga. USA
Registered: Jul 2013


Feedback score: (3)
Leave feedback

Rate this member

Report this Post07-14-2015 12:43 AM Click Here to See the Profile for f85gtronSend a Private Message to f85gtronEdit/Delete MessageReply w/QuoteDirect Link to This Post
I read your other post. Take a compression tester and thread it into the 02 sensor bung on the exhaust. I think the number was no more than 4lbs pressure to test the exhaust flow.
Also, what are the fuel pressure numbers like? Specifically first key on, ignition off and then idle. The last is full quick revving.
Post your findings.

My thoughts are plugged cat, or weak fuel delivery.

[This message has been edited by f85gtron (edited 07-14-2015).]

IP: Logged
fierofinder
Member
Posts: 629
From: Battle Ground, WA
Registered: Apr 2007


Feedback score: (1)
Leave feedback

Rate this member

Report this Post07-14-2015 01:03 AM Click Here to See the Profile for fierofinderClick Here to Email fierofinderSend a Private Message to fierofinderEdit/Delete MessageReply w/QuoteDirect Link to This Post
I agree sounds like plugged cat. Also check plug wires for excessive resistance.
IP: Logged
Jim_Martin29
Member
Posts: 702
From: Marina, California, USA
Registered: Jun 2010


Feedback score: N/A
Leave feedback

Rate this member

Report this Post07-14-2015 02:13 PM Click Here to See the Profile for Jim_Martin29Send a Private Message to Jim_Martin29Edit/Delete MessageReply w/QuoteDirect Link to This Post
I built a back-pressure test kit using an old O2 sensor, pressure hose, and a low pressure guage (0-5 psi).

I measured the back pressure and it did not go above 1.5 psi until I reved the engine to about 3000 rpm. I did manage to get it to 3 psi but I had to rev it to about 4000 rpm.

the cat is only a couple of years old and is a Magnaflow unit.

I tested the fuel pressure with a fuel injection test kit from O'Reilly's Auto. I don't remember the exact numbers (they are in my shop log at home) but the pressure was well within specs according to the shop manual.

Fierofinder, as mentioned in the previous post, the ignition system has been completely replaced and tested, new distrubuter, coil, plugs and cables. Also, the TPS, MAP, IAC, O2, and Incoming Air Temp Sensor have all been replaced with no change. (I decided to just replace all of the 30 year old sensors).

The car starts fine with no hesitation and runs fine at low speed without miss-fires or stumbling, although the idle did seem a little rougher this weekend and was not as smooth when downshifting (it's a manual trans). It's when I try to accelerate hard, like getting on the freeway or passing, that I have real problems.

When it was running correctly I could beat my wifes Audi up the on-ramp. Now I have to put the throttle peddle all the way on the floor just to get on the freeway at 50 mph.
IP: Logged
Jim_Martin29
Member
Posts: 702
From: Marina, California, USA
Registered: Jun 2010


Feedback score: N/A
Leave feedback

Rate this member

Report this Post07-14-2015 02:22 PM Click Here to See the Profile for Jim_Martin29Send a Private Message to Jim_Martin29Edit/Delete MessageReply w/QuoteDirect Link to This Post
I agree completely with the idea of weak fuel delivery. That is why I'm planning on taking a week of vacation (like I have a choice . . . I have to use 40 hours of vacation by the end of July, I have been told) and replace all of the injectors.

People have been telling me the injectors never wear out but after some research on here I see they do sometimes have problems. After 30 years of fuel and heat I'm sure there is varnish built up and perhaps some injectors are semi clogged.

I can't seem to come up with any other cause.

By the way, yes, I replaced the fuel filter (although it was only a few years old) when this whole thing started.

And, thanks, I do appreciate the help and suggestions.
IP: Logged
Patrick
Member
Posts: 30117
From: Vancouver, British Columbia, Canada
Registered: Apr 99


Feedback score: (1)
Leave feedback





Total ratings: 442
Rate this member

Report this Post07-14-2015 02:59 PM Click Here to See the Profile for PatrickClick Here to Email PatrickSend a Private Message to PatrickEdit/Delete MessageReply w/QuoteDirect Link to This Post
An EGR valve that's not closing properly (and/or opening too soon/too long) can cause some weird symptoms.

With it being relatively easy to do, how about blocking the EGR tube (I'd do it at the manifold end to also rule out a possible leaking tube) and see if temporary elimination of the EGR valve changes what you've been experiencing.

 
quote
Originally posted by Jim_Martin29:

replaced the EGR valve and the EGR solenoid


Installation mistakes can be made, parts can be faulty. Temporary elimination of the EGR system might shed some light on the issue. If nothing else, it eliminates a variable.

[This message has been edited by Patrick (edited 07-14-2015).]

IP: Logged

next newest topic | next oldest topic

All times are ET (US)

Post New Topic  Post A Reply
Hop to:

Contact Us | Back To Main Page

Advertizing on PFF | Fiero Parts Vendors
PFF Merchandise | Fiero Gallery | Ogre's Cave
Real-Time Chat | Fiero Related Auctions on eBay



Copyright (c) 1999, C. Pennock