Ok guys, I've been chasing this problem for a few months. Once my truck reachs around 80km/h in 2nd gear under hard throttle it starts to surge. I ran my codes, nothing stored and nothing related to the fuel or igniton system. It also does it under hard throttle above 110km/h. Any suggestions or idea's? Its the stock 1988 5.0/SEFI and a C6 tranny. The trucks runs great, but when its time to drop the hammer- it just surges.

 

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my guess would be a vacumn leak somewhere, i had an older car that would sit and idle like a champ,run so smooth you could balance a can of soda on it, ran great under mild loads like city driven, but when you punched it would suge from 1500 rpm to 3000 , up and down and up and down. came to find out that the intake manifold gasket was bad...changed it...problem solved...might be something you would wanna check ...good luck

 

It sort of surges or stumbles. I would say it only surges up and down maybe 300-500rpm above 80km/h in 2nd and above 110km/h in 3rd. Its just annoying compared to the way the 5.0 HO in my 63 Mercury hauls the mail without a hitch.
That ol' holley 600 works A1.

 

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Ok, I tested my MAP sensor and oxygen sensor. The output signal from the Oxygen sensor is .030-.040 so thats within spec (.035-.055). My MAP sensor is putting out 4.91 volts, spec is 5.0 volts. I couldn't do a vacuum test on it because I don't have a vacuum pump to bring it to 20 inches HG. I did notice my fuel return line is kinda kinked in a few places. My boss has a EEC Pro-Link so I'm going to borrow it and go for a drive to see what it say's when the surge/stutter occurs. I'm getting spark knock/pinging at the same time the stutter occurs, coincidence?
Any other suggestions or comments?

 

I don't know if this is the same problem but it sounds similar. I have a 1996 Ford F250 and when the engine was under a load it would surge.(351W) I couldn't figure it out and took it in to a very reputable shop. He checked everything out and looked okay. When he test drove it with the test equipment hooked up he said the oxygen sensor was lazy. It looked good but was slow. Replaced that and fixed. Hope this helps. Matt

 

I guess I'm between a rock and a hard place, I can get a new oxygen sensor for $67 CAN and try that but if that doesn't cure it then I wasted $67......ugghhh....I HATE EFI!!!!
I need this fixed but I'm too cheap to go to a Ford dealer when I know I can fix it myself and I will learn something along the way.

 

You absolutely have a vaccum pump - your engine.

Start it up, drive it around to warm it up, then shut it down, attach your vaccum gauge to the engine, and fire it up and let it idle.

At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 23" or more, then restablize at the original reading.

With the engine idling, continued fluctuation of 1 to 2 inches may indicate an ignition problem. Check the spark plugs, primary ignition circuit, high tension cables, distributor cap or ignition coil. Fluctuations of 3 to 4 inches may be sticking valves.

Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, manifold-to-plenum gaskets, vacuum brakes or the vacuum modulator, or vacuum cruise control motor or issue with the heating/AC controls. Low readings could also be very late valve timing or worn piston rings.

Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present. Check for bent tailpipes or a "stuffed" cat converter, the most common sources.

With the engine Idling, the vacuum gauge pointer will drop sharply, every time a cylinder leak occurs if the problem is with one cylinder, rather than an overall problem. The drop will be from the steady reading shown by the pointer to a reading of 10" to 12" Hg or less. If the leak Is between two cylinders, the drop will be much greater. You can determine the location of the leak by doing a dry/wet compression test.

Hope that helps.

 

hey!! change your fuel filter if you havn't already done so. try to blow through the old one i've seen some of these things so damn clogged it's almost impossible for gas to get through.. if that doesn't fix it check the tps it might have a flatspot towards wot. best to start with the simple things. good luck

 

The fuel filter looks like it was changed shortly before I bought the truck in July, it looked fresh right out of the box. I need a vacuum pump according to my Haynes manual to test the MAP sensor, I already have good engine vacuum. I need to see what happens when 20HG of vacuum is applied to it, the book says it should drop to around 1 volt from the initial 5 volts. I wonder if the TP sensor could cause the stuttering problem as suggested, I never checked that yet. I've been going through each possibility one thing at a time. Isn't it funny how a carburetor can do all these jobs on its own with far fewer parts?

I'll try your suggestions tommorow as I plan on friggin'with the truck for a couple of hours if its not too cold outside.

 

if you ask me a carburator is way more complicated than efi. well some of the older ones and aftermarket ones are simple but the last carburators they used are a real pain mainly because of the 5 milles of vacuum lines. i thought most map sensors output a frequency instead of voltage. if you get a vacuum guage check out this link http://users.bigpond.net.au/ergoff/vac1.htm
it tells you everything you need to know.

 

According to my Haynes Manual I should see a voltage drop. I just bought a pretty fancy OTC Stinger multimeter that has a lot of features, I just have to figure them all out and my old truck is a good place to do it. The thing even has a tach built into it, best $140 I ever spent.

 

While I agree that troubleshooting EFI can be a pain at times, there are so many advantages to EFI over a carb I wouldn't EVER consider going backwards.

Turn the key, the vehicle starts. No pumping, no flooding, no choke problems.

Starts when its cold.
Starts when its hot.
Starts when its humid.
Starts when its dry.

EFI can tolerate leaky valves, the timing being off, leaky heads, severe rod knock, bad emissions equipment, and even low vacuum to some degree.

I was discussing this with a friend recently actually, he commented on how 70's engines out of junkyards usually run pretty good based on what he's pulled over the years, whereas later model engines he's pulled have not, and required more R&R to get things going properly.

I had suggested that maybe because EFI is so "smart", the uninformed driver thinks because the car goes, nothing is wrong, despite the rod knock and odd clangs.

Maybe I'm biased. I have never EVER in my life been able to tune anything with a carb and have it run right. I fully respect anyone with a magic screwdriver

 

I fully respect anyone with a magic screwdriver



same here