drop in idle

[90broncoman]

Having this issue for a long time, and kind find many answers.

Got a 90 ford bronco with 5.8 new motor. when at idle the rpm spikes down every few sec, about 200 rpm or so. while driving under load it acts like the engine shut of for a split sec. once and awhile at a stop sign or red light it will die like its out of fuel or something. replaced just about every part:

Fuel pump

Coil pack

Spark plugs and wires

Distributer

tested with different map sensor

IAC

EGR

tired of throwing parts at it. wish I could up load a video to show y'all what its doing. found a similar symptom on youtube:

I thought of fuel pressure regulator or computer.

also when I disconnect the map sensor it goes away. but I know that puts computer into a default setting.

what y'all think?

thanks

 

[miesk5]

yo 90,

WELCOME!

Curb Idle and Fast Idle are controlled by the PCM and IAC and are not adjustable.

Engine Load, TPS Sensor and the IAC are very closely linked, they must all work together to maintain a good idle.

Try a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19

http://broncozone.com/topic/14269-code-reader/page__pid__74587__mode__threadedThe engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.

Run it around to heat the engine up and shift thru all gears incl Reverse. Then turn off all accessories/lights, etc.

Make sure A/C is off and transmission is in Park (automatic) or in Neutral for a Manual & release clutch.

Do Key On Engine Off (KOEO) portion first.

And Post em here according to:

KOEO

&

KOER (IF YOU CAN GET IT RUNNING LONG ENUF TO HEAT ENG UP)

try unplugging the IAC motor. If the idle drops, its usually due to an input signal to the computer which results in the computer raising the idle speed. If this is the case, look at all of the computer inputs to make sure everything is up to par. If the engine RPM does not drop when the IAC motor is disconnected, then either the IAC motor is not functioning properly, or, there is a vacuum leak in the engine. Stop the engine, remove the IAC motor and block off the passages. Restart the engine to see if there is any change in engine RPM. An idle speed higher than base idle specs indicates a vacuum leak. If the engine RPM is at base idle specs, then the IAC motor is bad. Resistance specs are 7-13 ohms. A good resistance reading does not necessarily mean that the IAC motor is good. Occasionally, an IAC motor will become weak, and will actually bypass too much air, causing a high idle condition even though everything else is operating properly.

IAC Testing & Cleaning by Seattle FSB

My link

Consider Ford Fuel Injection and Electronic Engine Control: How to Understand, Service and Modify, 1988-1993; & Ford Fuel Injection and Electronic Engine Control: How to Understand, Service, and Modify All Ford-Lincoln-Mercury Cars and Light Trucks, 1980 to 1987 by Charles Probst

avail @ BARNES & NOBLE, AMAZON, E BAY, ETC.

see Part 9 Tuning for Performance

 

[90broncoman]

no codes. just the 111 system pass. to me it feels electric. but what would cause that and then all be fine if map sensor is unplugged?

 

[miesk5]

yo,

fuel pressure regulator - pull vacuum hoise off; any gas or aroma of, replace FPR bec its diaphram ruptured.

When the vacuum hose to MAP is disconnected, PCM sees wide open throttle and runs in full rich, as you know.

If Connector is disconnected, PCM stores a DTC and go into into a Failure Mode Effects Management (FMEM) strategy where it assumes values based on other sensors such as the TPS.

Did you do KOER test as well as KOEO?

Anything that interferes with the MAP sensor's ability to monitor the pressure differential may upset the fuel mixture and ignition timing. This includes a problem with the MAP sensor itself, grounds or opens in the sensor wiring circuit, and/or vacuum leaks in the intake manifold (airflow sensor systems) or hose that connects the sensor to the engine.

A vacuum leak will reduce intake vacuum and cause the MAP sensor to indicate a higher than normal load on the engine. The PCM will try to compensate by richening the fuel mixture and retarding timing -- which hurts fuel economy, performance and emissions

The vacuum gauge should show around 18-22 in-Hg @ idle & maintain a steady needle. Rev it up to ~2500 rpm, hold it steady, and you should see the needle drop, then slowly increase to a level close to the idle reading (though slightly lower is normal).

Inspect Vacuum hoses & vac operated components;

Vacuum Leak Test @ http://broncozone.com/topic/22770-low-idle/

here is neat Vacuum Gauge http://www.autoparts2020.com/rsdev/part_detail.jsp?PART_HDR_ID=87069

Gas Saver Vacuum Gauge

 

[90broncoman]

didnt get any smell from fpr. but i replaced it anyway due to a suggestion. no change.

i am getting around 18 vaccuum, and when it reves up like you said it does drop and then climb again.

Still at loss what this issue may be. im almost leading to the computer.

any other checks you can suggest?

thanks for al the help!

 

[miesk5]

yo 90

I can only suggest the Probst Table I posted earlier as an aid on what else to check/test

Although you have just replaced the IAC, Ford put this TSB out;

Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others; "...Hard cold starts, hesitation and stalls on initial start-up or during idle or decel may be caused by sludge in the throttle body and/or idle by-pass valve. Sludge deposits or oil film on the throttle body bore and plate or the idle air by-pass valve may cause one or more of the following conditions. Hard Cold Start, Stall On Initial Start-Up, Stall During Idle, Stall During Decel, Rough Idle, Rolling Idle, Hesitation During Acceleration. A new idle air by-pass service kit (F2PZ-9F939-A) is now available for service use to correct sludge contamination concerns of the throttle bore and plate only. It eliminates the need to clean the majority of past model throttle body applications. Cleaning is not required on sludge tolerant throttle body designs released for 1991 and newer model years..."

http://www.supermotors.net/registry/media/724437_1

Nelbur posted a diy here;

Idle Air Control (IAC) Modification in an 86; "...I have spent some time this week trying to set up an air bypass around the IAC valve by cutting away some of the gasket between the IAC in and out air holes, rather than pay big bucks for Fords spacer kit. I cut away the center of the original IAC gasket from the outside of each hole to the outside of the other hole, giving about 1/2" gap for the air to pass through. I noticed an immediate improvement in the engines starting behavior, but it would still die occasionally. For most of my trucks life it has been dying when it was started hot. It would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator. It would never die when cold as the IAC would keep the idle speed up, and it never died at stop lights. Now it's worst situation seems to be when it has been shut down for a half hour or so, and restarted. I wonder if the heat soak is fooling the ETC into thinking the engine is warmer than it really is. I decided that more improvement could be had if I had a thicker gasket, because the original IAC gasket was very thin (0.018"). I had some 0.030" gasket material so I made my own with the same 1/2" cut out. This gave enough bypass to noticeably raise the idle speed and almost eliminate the dying. After maybe 50 starts in the last few days, it only died twice. After so many years of catching it with the accelerator it is darned hard to leave my foot off it. It is clear to me that by trial and error one can tune the air bypass without the need for the expensive Ford kit. I may combine the two gaskets for more bypass, but the idle is about as fast as I would want now, especially for driving in snow..."; miesk5 Note, Tomco makes an idle plate P/N 8491 = to Ford, P/N F2PZ9F939A; Like Ford’s service kit, Tomco’s spacer plat e (arrow) is installed between the air bypass valve and its mounting boss using two gaskets. The two holes in the plate are for bleeder screws, which provide a minimum air rate adjustment independent of the throttle opening.

Source: by Nelbur at Ford Bronco Zone Forums

http://broncozone.com/topic/22150-fast-idle-problem/page__pid__115710#entry115710

Symptoms were; "the Bronco is running ok after replacing the alternator except for idling. The problem now is the fast idle isn't kicking in when I start her up and the engine is cold. It would usually idle fairly high then tone down with no idle issues at all. It stalled on me for the first time today coming home from work idling in traffic. I just got her back on the road now she's not idle correctly...

The idle is also "hunting" when it starts to warm up (some times cold) and it never used to do that. It's putting a strain on the electrics when it surges up/down. I can watch the voltage drop as the idle drops, then the gauge goes back to normal voltage as the loping idle hits its high point..."