'90 EFI 351

if a very small amount of throttle is applied, the engine seems to want to stall especially when cold. It does stall occasionaly if i dont let it warm up for a few minutes. If im coasting from a stop at idle when still not warmed up and lightly touch the pedal it goes down the street sputtering on its own with my foot off the pedal. It sounds like when you disconnet the map sensor. Once its warmed up it seems ok and runs great but there seems to be hesitation all the time at very low throttle and its not

Runs great above those low speeds.

I almost feel like its flooding at low speeds cause my gas mileage has been around 10MPG city 17-18Mpg highway(whcih is great). I thought ive smelled fuel a few times but not always.
Would a exhaust backpressure be causing this problem? . Cat? Ive had it looked at by several garages and all say my exhaust is ok(didnt actuall look at my cat) but im sure i hear exhaust leaks but cant find them.


Ive changed the fuel pressure regulator several months ago and TPS, IAC, O2, (plugs, rotor, wires.....)within the last three months on unrelated problems.

There are no codes either KOEO or KOER(which is great) ive solved a lot of problems with help from people on this board.

Any ideas would be appreciated

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If you can HEAR what sounds like an exhaust leak but can't find it, check the Thermactor check valve in the air injection crossover tube that runs between the back of the heads. This check valve is notorious for cracking in half and allowing exhaust to escape back through the air injection ports in the heads. It will also cause some of the hesitation you mention because of the lack of thermactor air injection at idle and cold starts. Its buried way back in the engine bay right against the center of the firewall.

When was the last time the IAC was cleaned? Another possibility would be a reduced amount of idle air causing a rich condition at idle/low end. You won't get any codes on it because the computer will compensate for it. This combined with the possible loss of sir injection to the heads would do all sorts of things to idle and low end running.

 

Check your header bolts, a leak at the headers at the exhaust manifold will give this condition.

 

Ill check these out today. On a related note, I noticed this morning after i start it then turn on the air conditioner, it idles from 800RPM to 1600RMP every 3-4 seconds for a minute or 2. After a few minutes when idled down to ~850RPM then i turn on the air conditioner it drops to 600 RPM and wants to stall then goes back to normal.
Is this a vacuum leak somewhere? maybe theres no vacuum temporarily so it does this till it gets it back.? Im just guessing.

thanks

 

Thats sounds even more like a dirty IAC and maybe an intermittant vacuum leak as well. Although the rpms sound close to being right. The A/C being switched on should send a signal to the EEC computer to up the idle slightly however, if it cannot control the IAC or thermactor system, the engine is at the mercy of whatever drag is applied to it. You might try running the KOER (Key On Engine Running) test and see what codes pop up.

 

'90 EFI 351

Concerning the exhaust check valve, i do have a lot of black soot on the firewall passenger side where the check valve is. I wiped off part of it 2 months ago and it hasnt come back. I have looked at the metal tube and air injection rubber tube from the diverter valve (with upper air intake manifold off) and it seems all connected and ok. Is it possible the valve is not working, ie. exhaust is coming up , but doesnt appear busted?
I dont see any smoke or smell anything. The vacuum hoses going past there do seem to be working. Also whenever i turn my heater on, or worse air conditioning, the rpms drop to 400rpm temporarily and she wants to stall, then they level out fluctuating slightly at about 600 rpm while idling.

Ive replaced the IAC valve with a cleaned older one I had and it does run better when i just touch the gas and doesnt want to stall but still hesitates a lot.

IM thinking of trying to replace the check valve this weekend. How about the other check valve running from the diverter valve to the pipe that goes between the 2 cats.? Can exhaust back up from there?

Its seems very hard to get hands back there but I assume it is possible without removing the engine!


Thanks for any info.

 

Yes, the valve can go bad and allow exhaust to back up through the thermactor system. BOTH check valves are there to keep this from happeneing. If either or both are malfunctioning there is a danger of damaging the thermactor system hoses and diverter valve. I recently had to replace the one in the crossover tube between the heads on my 93. It proved easier to pull the two bolts holding the crossover tube into the heads and remove the entire crossover tube with the dead valve still attached to it. I then was able to replace the valve quite easily and reinstall the crossover tube. the one on the cat can be slightly more difficult if the OEM metal tube has rusted or "grown" to the bottom of the check valve. This usually requires cutting the tubing, replacing the check valve and buying a short length (a foot or less) of high temp "emissions" hose to span the gap. Or you can weld a new section of tubing in when you replace the valve.

 

I will be watching this one..my 90 is doing the EXACT same thing and it's been driving me nuts.

 

iT WAS BAsically the IAC valve. I got a new (used) throttle body and used the one on there. Must have been bad. I put my old IAC on and it went away.

 

just out of curiousity, what would happen if the thermactor system was not there? I mean if someone were to take the time to put resistors and whatever needed to tell the comp that the system is workin properly.