one of my friends has a 90 ford bronco with a 94 mustang engine in it, all the fuel injection and related parts are from the 302 out of the bronco, it'll idle fine for a while then it starts surging rev's up then done to where it almost quiets, it has all new sensors, tuneup, changing the 02 sensor tonight, it had a egr code they replaced that and it still does it, has anyone had this problem ?

any help would be appreciated...

 

Not sure which sensors have already been replaced, but I would check/clean the IAC valve. Also wouldn't hurt to check the TPS voltage.

 

it has all new sensor's , the 02 sensor is being installed right now,

 

I have a roller cam Mustang engine in my '86 Speed Density FI Bronco - I found that the idle stop screw on the throttle body linkage bracket (whew!) has to be in just the right spot, or the idle will run up and down. IT does this if you get the idle too low or too high, so you'll have to fiddle with it to find the sweet spot. I wanted to raise the idle speed because of another minor issue related to the HO roller engine, but it didn't like it. My Bronco runs about 1500rpm upon cold start up, and after 45 seconds or so it drops down to 750-800 or so. The "other issue" is a stall if I give it gas to accelerate and then snap the throttle shut immediately - rpm's drop low, then overcompensate high before normalizing. Sometimes it's enough to stall the engine. My theory is that upgrading your Speed Density FI to Mass Air FI will solve the problem. It can be done by replacing the injector harness and PCM with an HO unit. It requires some "gnat's a5s" wiring though - don't try it without a wiring diagram. I have the diagrams, but I've not gathered the pieces yet. Anyone else done the electrical side of the HO swap?

 

Oh yeah - my IAC is squeaky clean, too.

 

As far as the EGR code goes, you may need to clear the ECM Code after installing the new one. The ECM will still show the bad code even though the problem is fixed.

I have a similar problem that has given me fits on my 90 Bronco w/5.8. Idles fine sometimes, then will go up to 1500 RPM? I get no codes either.
JD

 

I want to change over my H.O. to mass air also. I am totally clueless on how to do it though. What type of diagrams do I need? My H.O. runs great with no problems at all(so far with 5000 miles) but I was told that mass air would give me better gas milleage and power. Just something I would like to do down the road.

 

I also have the 90 with the 351, and it does the same thing... It idles just under 1500 at first, then drops down till it almost stalls, then surges up again... I've been wondering if there's a circuit to monitor the engine's output to the battery? Was a blind hunch though, just noticed that it kept the low idle until the voltmeter dropped, then it shot back up again... Never claimed to be a mechanic, but I figure it's a thought?

Edit: and my idle has never been adjusted from stock, and it doesn't do it when it's warm, and it has a new TPS, not sure on IAC though.