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Chevy S10 Engine Problems

1995 Chevy S10 4.3 V6 TBI VIN Z engine 238k miles.

If anyone remembers I had my engine replaced with a used one little over a year ago. I had engine stumbling with light throttle just off of idle. The engine had a coolant leak internally and the EGR was sticking according to the mechanic. He just cleaned the EGR valve then but I replaced it later.

Anyway, I started getting similar problems again last week. The engine was shaking and stumbling when I give it throttle from idle. It was idling fine and would smooth out at higher rpm, maybe less power though.

So today I was out checking the EGR valve again. Took it out, put it back, then I started checking the vacuum hoses off the EGR solenoid.

I put my finger over one of the hose ends or maybe the vent, not even sure. Then I get a hissing sound from the throttle and the engine idle drops and it starts stumbling really bad. Connected the hoses back and it’s still hissing and idling terrible.

Turned engine off and it has a little smoke coming from the IAC valve area inside the throttle. I went and bought a new IAC valve, installed and it’s still the same. Noticeable hissing from throttle, I think it’s coming from the valve port area. Hiss is hard to hear with the air cleaner back on. Always has a little smoke coming off the valve port area when I shut the engine off.

I’ve never taken the throttle body off, I’m not sure what to be looking at now. Maybe the valve isn’t getting a signal or I have an air leak somewhere else. Does anybody have suggestions? Thanks.

If the engine was replaced with a used one, the problem might be with the original spider assembly.

This fuel injection system uses fuel pressure to blow open the poppet valves.

First check the fuel pressure on this system. Because it relies on at least 63 PSI to operate.

If the fuel pressure is good, then you’re looking at replacing the spider assembly.


Mine has the old fashioned TBI with just the 2 injectors inside the throttle.

I should add I never got any CEL. It was strange, right when I blocked the vacuum with my finger, it was like something gave out and the hissing and stumbling started instantly. They had a reset procedure for the new IAC, driving at 40 mph on the road, I did that and didn’t help.

1995 GM

Non-compliant OBDII


If you are hearing a “hissing” sound with all the vacuum hoses connected properly, you got to figure out what’s causing it. B/c that almost alwlays indicates a vacuum leak is present, and that will definitely cause poor idling and stalling out. It usually isn’t a problem at higher speeds b/c there’s already a lot of airflow into the engine then, so a little more doesn’t hurt. But at idle , b/c there is so little airflow in that condition, any add’l airflow getting into the engine in a manner it isn’t supposed to, will usually cause stumbling. The source of the vacuum is usually the intake manifold, and hoses go to various places which then use the force of that vacuum to open valves. But there isn’t supposed to be any airflow into the intake manifold those hoses, just a way to transfer of the vacuum force. And each of those had a rubber diaphragm in it, which can break, and create a vacuum leak and associated hissing. Your shop will need to test them one by one until they find the leak. I do this job myself using an inexpensive hand-held vacuum pump gadget.

There’s other things that can cause engine stumbling at idle, but that’s the place to start for your problem imo.

It’s OBD I system on the sicker, It has an OBD II connector though. I have a scan tool for it which shows some stream data.

when you plug in your scan tool, are you able to monitor fuel trims?

If so, please tell us fuel trims at idle

Put the air cleaner on and start it back up. If the noise is now gone I’ll bet the hissing is the IAC opening up as wide as it can to try and keep engine running. Which means you need to chase the poor running problem so the IAC can return to normal operation.

So I went out today to mess with it again and it starts right up and idles normal now. It behaves fine until it warms up fully then it’s just doing the original problem I had in the first place. It shakes and stumbles just giving it light throttle at idle. It revs fine at higher rpm and idles fine.

I couldn’t find any vacuum leaks. I tried plugging the egr valve then unplugging the solenoid and no change.
Then I unplug the MAP sensor. It idles slightly rougher but then I tried tapping the throttle from idle and it doesn’t stumble at all. I plug it back in and after a couple minutes it does the stumbling again. I repeat the process a couple times and it doesn’t seem to stumble with the MAP unplugged.

I tried to test it with a multimeter but my probes are too short to reach in the harness. I ordered another MAP anyway. I drove it around the block, seems to lack power but it’s not doing the stumbling. Is it okay to drive short distance with the MAP unplugged?

For the future - I keep a couple paper clips with my meter; Stick the end in a connector, and alligator clip one lead of the meter to it. Be careful on live voltages if the two are next to each other.


The ECU will use default, compromise values with the MAP disconnected because of the lack of input. You could try driving short distances, but be careful of any situation where it would be dangerous to stall out (think railroad crossing).

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That’s a good idea, I’ll try that tomorrow. It’s really cold and I’m doing this outside so my patience runs out fast.

Good suggestion. I’m not going to drive it very far.

MAP failure reports are not common here. I doubt this is caused by a faulty MAP sensor. It sounds like it is running too lean at idle to me. It doesn’t show up first start of the day t b/c the computer automatically enriches the mixture until the engine warms up, then leans it out. When it leans out, that’s probably when the stumbling at idle starts. So you either got too much air going in, or too little gas being injected.

EGR problems are sort of like too much air going in, so it could still be that. The tests you’ve done are good ones, but not sufficient to prove the EGR system is working. What I do on my Corolla is apply a vacuum to the EGR valve using a manually operated vacuum pump. At warm idle. If the EGR valve is doing its thing, the engine will stumble badly and usually stall.

Some kinds of vacuum leaks are very difficult to find. Like leaks from outside air directly into the intake manifold. Have you tried the method of spraying starter spray around the intake manifold/throttle body gasket areas and other places a vacuum leak might occur? If the engine speeds up when you spray in a certain area, that is consistent with a vacuum leak in that area.

You may be right. I don’t have a vacuum pump or a gauge. I did test the egr valve off the car, it holds pressure and seats properly. Whether the solenoid is functioning properly, I don’t know. I did spray carb cleaner all around and didn’t pick up anything. I was thinking it may be the fuel pressure regulator and not a vacuum problem.

Check for manifold vacuum at the map sensor vacuum port. Have seen these ports get plugged up causing the map sensor readings to be askew. If it is plugged up, get a throttle body gasket and remove throttle body to clean out the gunk from passage. Of course, if you have shop air you may be able to blow out the vacuum port to clear it out.

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So it wasn’t the MAP sensor. I changed the fuel pressure regulator and that’s not it. It actually runs worse now with the new regulator. The spray looks better but didn’t help. It’s really bogging down going up a hill or lugging the engine at all. I might buy a fuel pressure gauge just to confirm that. Anyway, I made an appointment at the place that put the engine in for next week.

Have you tried just disconnecting the EGR valve from its electrical and vacuum inputs? It’ll probably throw a code, but if the engine runs better with the EGR valve disconnected, you’ll at least know there’s some EGR involvement.

I tried plugging the egr vacuum line and the electrical connector, it didn’t make any difference and didn’t throw a code. Mechanic said it could be the temp. sensor since it acts up after warming up. It seems to be getting worse every time I drive it. It still idles fairly normal, maybe a little rough.